<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:69d5aae9-2169-44ac-985d-3a45b457491f,g:b7d0d76a-879c-44ae-8c66-bff31b3e6768-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://www.magnite.com/role/ConsolidatedStatementsofStockholdersEquity" xlink:type="simple" xlink:href="mgni-20251231.xsd#ConsolidatedStatementsofStockholdersEquity"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/ConsolidatedStatementsofStockholdersEquity" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_d74702d9-533e-452d-bd96-e9cbd9e9aa29" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_31d51a7c-96a4-4bb3-8b57-2517e2ce742c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_d74702d9-533e-452d-bd96-e9cbd9e9aa29" xlink:to="loc_us-gaap_StatementTable_31d51a7c-96a4-4bb3-8b57-2517e2ce742c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_24f9b4a7-a1b7-4627-841e-4225f8f6e916" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_31d51a7c-96a4-4bb3-8b57-2517e2ce742c" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_24f9b4a7-a1b7-4627-841e-4225f8f6e916" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_24f9b4a7-a1b7-4627-841e-4225f8f6e916_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_24f9b4a7-a1b7-4627-841e-4225f8f6e916" xlink:to="loc_us-gaap_EquityComponentDomain_24f9b4a7-a1b7-4627-841e-4225f8f6e916_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_bd2888e4-7353-41d5-9bd0-595850fa48e0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_24f9b4a7-a1b7-4627-841e-4225f8f6e916" xlink:to="loc_us-gaap_EquityComponentDomain_bd2888e4-7353-41d5-9bd0-595850fa48e0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_783de895-aa86-4718-a692-d7b33c19eeb6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_bd2888e4-7353-41d5-9bd0-595850fa48e0" xlink:to="loc_us-gaap_CommonStockMember_783de895-aa86-4718-a692-d7b33c19eeb6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_747eebc7-7e05-4a32-b454-ee4c70140bbc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_bd2888e4-7353-41d5-9bd0-595850fa48e0" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_747eebc7-7e05-4a32-b454-ee4c70140bbc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_934c46e1-af6c-4df2-95b3-f22f5cb9db52" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_bd2888e4-7353-41d5-9bd0-595850fa48e0" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_934c46e1-af6c-4df2-95b3-f22f5cb9db52" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_ab8f29c5-2f64-46cb-ae22-4b13c7727b88" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_bd2888e4-7353-41d5-9bd0-595850fa48e0" xlink:to="loc_us-gaap_RetainedEarningsMember_ab8f29c5-2f64-46cb-ae22-4b13c7727b88" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockCommonMember_a572e835-20f1-4ca0-94d5-f87a962b9de3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TreasuryStockCommonMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_bd2888e4-7353-41d5-9bd0-595850fa48e0" xlink:to="loc_us-gaap_TreasuryStockCommonMember_a572e835-20f1-4ca0-94d5-f87a962b9de3" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_ed6f975b-4c77-4c23-b723-a8f6a566553a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_d74702d9-533e-452d-bd96-e9cbd9e9aa29" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_ed6f975b-4c77-4c23-b723-a8f6a566553a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_a39ec012-f908-4b0e-ac0e-866cc5b324bc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_ed6f975b-4c77-4c23-b723-a8f6a566553a" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_a39ec012-f908-4b0e-ac0e-866cc5b324bc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_56bdd0b1-a801-47fc-acf0-f27e0e81b31d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_ed6f975b-4c77-4c23-b723-a8f6a566553a" xlink:to="loc_us-gaap_StockholdersEquity_56bdd0b1-a801-47fc-acf0-f27e0e81b31d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockCommonShares_aa31ace9-a722-45f0-99aa-676b8abf6e4e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TreasuryStockCommonShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_ed6f975b-4c77-4c23-b723-a8f6a566553a" xlink:to="loc_us-gaap_TreasuryStockCommonShares_aa31ace9-a722-45f0-99aa-676b8abf6e4e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_c96ae57d-11ad-4406-9057-71cd86f5dc5a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_ed6f975b-4c77-4c23-b723-a8f6a566553a" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_c96ae57d-11ad-4406-9057-71cd86f5dc5a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_9399953d-9bbc-4710-96cf-0a6e5c3a8dcc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_ed6f975b-4c77-4c23-b723-a8f6a566553a" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_9399953d-9bbc-4710-96cf-0a6e5c3a8dcc" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_d4d58a35-e49f-4866-b524-819c478ab09e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_ed6f975b-4c77-4c23-b723-a8f6a566553a" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_d4d58a35-e49f-4866-b524-819c478ab09e" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_f5fece29-78f0-4f43-84ca-f8da5e291915" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_ed6f975b-4c77-4c23-b723-a8f6a566553a" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_f5fece29-78f0-4f43-84ca-f8da5e291915" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_07a6c6e8-ccde-429e-8178-049e1a41ff0f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_ed6f975b-4c77-4c23-b723-a8f6a566553a" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_07a6c6e8-ccde-429e-8178-049e1a41ff0f" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation_ada0a4a4-32ec-42c2-98fa-7edb875dab5f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_ed6f975b-4c77-4c23-b723-a8f6a566553a" xlink:to="loc_us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation_ada0a4a4-32ec-42c2-98fa-7edb875dab5f" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation_0d830f34-2c61-4a2c-8f37-6925e714f415" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_ed6f975b-4c77-4c23-b723-a8f6a566553a" xlink:to="loc_us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation_0d830f34-2c61-4a2c-8f37-6925e714f415" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockSharesAcquired_019bb625-8732-7c2f-86be-ab12bf501c97" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TreasuryStockSharesAcquired"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_ed6f975b-4c77-4c23-b723-a8f6a566553a" xlink:to="loc_us-gaap_TreasuryStockSharesAcquired_019bb625-8732-7c2f-86be-ab12bf501c97" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockValueAcquiredCostMethod_5bb8a7a5-1f23-4688-a963-14654b455c95" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TreasuryStockValueAcquiredCostMethod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_ed6f975b-4c77-4c23-b723-a8f6a566553a" xlink:to="loc_us-gaap_TreasuryStockValueAcquiredCostMethod_5bb8a7a5-1f23-4688-a963-14654b455c95" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_34bc2933-283c-432d-9a82-e95a1fdf3dce" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_ed6f975b-4c77-4c23-b723-a8f6a566553a" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_34bc2933-283c-432d-9a82-e95a1fdf3dce" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_5a0a657c-fcf6-4f08-83b9-788cd171dd3a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_ed6f975b-4c77-4c23-b723-a8f6a566553a" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_5a0a657c-fcf6-4f08-83b9-788cd171dd3a" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_bec50587-f070-48ff-9f61-c3b88fdaa20a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_ed6f975b-4c77-4c23-b723-a8f6a566553a" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_bec50587-f070-48ff-9f61-c3b88fdaa20a" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_2200ea4d-f9e0-4cef-9b7e-5a39dcd82f6b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_ed6f975b-4c77-4c23-b723-a8f6a566553a" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_2200ea4d-f9e0-4cef-9b7e-5a39dcd82f6b" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_8a21c915-d0e3-46fd-803e-935f0fddb564" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_ed6f975b-4c77-4c23-b723-a8f6a566553a" xlink:to="loc_us-gaap_NetIncomeLoss_8a21c915-d0e3-46fd-803e-935f0fddb564" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_087b4945-14b4-402b-aa24-acb59b4143d2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_1e8b84bf-0b08-41c3-8714-ba517e367601" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockCommonShares_afe08218-dca3-445f-8583-8147952ac33f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TreasuryStockCommonShares"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/ConsolidatedStatementofCashFlows" xlink:type="simple" xlink:href="mgni-20251231.xsd#ConsolidatedStatementofCashFlows"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/ConsolidatedStatementofCashFlows" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_4dcb27ce-9e98-4179-b1e4-37c3efc97eb6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_ea723278-59b6-445e-89c5-5f9a5b992635" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_4dcb27ce-9e98-4179-b1e4-37c3efc97eb6" xlink:to="loc_us-gaap_StatementTable_ea723278-59b6-445e-89c5-5f9a5b992635" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_cde934d7-b7a7-4dfc-a4e7-8d93173fcbad" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_ea723278-59b6-445e-89c5-5f9a5b992635" xlink:to="loc_us-gaap_DebtInstrumentAxis_cde934d7-b7a7-4dfc-a4e7-8d93173fcbad" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_cde934d7-b7a7-4dfc-a4e7-8d93173fcbad_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_cde934d7-b7a7-4dfc-a4e7-8d93173fcbad" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_cde934d7-b7a7-4dfc-a4e7-8d93173fcbad_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_1c516d5d-9f8e-40e2-8f61-402c02b8747a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_cde934d7-b7a7-4dfc-a4e7-8d93173fcbad" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_1c516d5d-9f8e-40e2-8f61-402c02b8747a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_CreditAgreements2021And2024Member_eef1326e-ab31-4e94-a39f-a1866bab3652" xlink:href="mgni-20251231.xsd#mgni_CreditAgreements2021And2024Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_1c516d5d-9f8e-40e2-8f61-402c02b8747a" xlink:to="loc_mgni_CreditAgreements2021And2024Member_eef1326e-ab31-4e94-a39f-a1866bab3652" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_15d468dc-3eef-46a5-b457-2244a03afa19" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_4dcb27ce-9e98-4179-b1e4-37c3efc97eb6" xlink:to="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_15d468dc-3eef-46a5-b457-2244a03afa19" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_593d4b7f-0f5c-4c43-870a-f3821e542d66" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_15d468dc-3eef-46a5-b457-2244a03afa19" xlink:to="loc_us-gaap_NetIncomeLoss_593d4b7f-0f5c-4c43-870a-f3821e542d66" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_fd343f53-24ef-4c36-a045-2cf25ce354c8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_15d468dc-3eef-46a5-b457-2244a03afa19" xlink:to="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_fd343f53-24ef-4c36-a045-2cf25ce354c8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DepreciationDepletionAndAmortization_aee69539-edfb-4e03-aa96-d10b01024f40" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DepreciationDepletionAndAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_fd343f53-24ef-4c36-a045-2cf25ce354c8" xlink:to="loc_us-gaap_DepreciationDepletionAndAmortization_aee69539-edfb-4e03-aa96-d10b01024f40" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensation_ec7da26e-d21c-4f8f-9157-6f1cf885377e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_fd343f53-24ef-4c36-a045-2cf25ce354c8" xlink:to="loc_us-gaap_ShareBasedCompensation_ec7da26e-d21c-4f8f-9157-6f1cf885377e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainsLossesOnExtinguishmentOfDebt_c332784a-3061-4204-a039-fd5595453a52" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GainsLossesOnExtinguishmentOfDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_fd343f53-24ef-4c36-a045-2cf25ce354c8" xlink:to="loc_us-gaap_GainsLossesOnExtinguishmentOfDebt_c332784a-3061-4204-a039-fd5595453a52" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProvisionForDoubtfulAccounts_1bfc4f8e-09e5-4093-8382-8b8ebffc3616" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProvisionForDoubtfulAccounts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_fd343f53-24ef-4c36-a045-2cf25ce354c8" xlink:to="loc_us-gaap_ProvisionForDoubtfulAccounts_1bfc4f8e-09e5-4093-8382-8b8ebffc3616" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfFinancingCostsAndDiscounts_4a8115bf-47ca-482d-8207-0156e9b883ab" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AmortizationOfFinancingCostsAndDiscounts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_fd343f53-24ef-4c36-a045-2cf25ce354c8" xlink:to="loc_us-gaap_AmortizationOfFinancingCostsAndDiscounts_4a8115bf-47ca-482d-8207-0156e9b883ab" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_NonCashLeaseExpense_83c7e225-0ef8-456c-b185-c947e92ef4d1" xlink:href="mgni-20251231.xsd#mgni_NonCashLeaseExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_fd343f53-24ef-4c36-a045-2cf25ce354c8" xlink:to="loc_mgni_NonCashLeaseExpense_83c7e225-0ef8-456c-b185-c947e92ef4d1" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredIncomeTaxesAndTaxCredits_ebc8d80f-6dd8-491f-b3dc-de8f8101ace8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredIncomeTaxesAndTaxCredits"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_fd343f53-24ef-4c36-a045-2cf25ce354c8" xlink:to="loc_us-gaap_DeferredIncomeTaxesAndTaxCredits_ebc8d80f-6dd8-491f-b3dc-de8f8101ace8" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignCurrencyTransactionGainLossUnrealized_b43da2c2-0598-4e2d-ab31-f79c750f7134" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ForeignCurrencyTransactionGainLossUnrealized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_fd343f53-24ef-4c36-a045-2cf25ce354c8" xlink:to="loc_us-gaap_ForeignCurrencyTransactionGainLossUnrealized_b43da2c2-0598-4e2d-ab31-f79c750f7134" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherOperatingActivitiesCashFlowStatement_104afa49-b6f9-450d-8ede-a8abc097a814" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherOperatingActivitiesCashFlowStatement"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_fd343f53-24ef-4c36-a045-2cf25ce354c8" xlink:to="loc_us-gaap_OtherOperatingActivitiesCashFlowStatement_104afa49-b6f9-450d-8ede-a8abc097a814" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_f57440ca-7f33-4a1d-8de2-2512c65d6882" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInOperatingCapitalAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_fd343f53-24ef-4c36-a045-2cf25ce354c8" xlink:to="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_f57440ca-7f33-4a1d-8de2-2512c65d6882" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInAccountsReceivable_b62d5247-347c-44d2-a1a6-8cadfb5508ac" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInAccountsReceivable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_f57440ca-7f33-4a1d-8de2-2512c65d6882" xlink:to="loc_us-gaap_IncreaseDecreaseInAccountsReceivable_b62d5247-347c-44d2-a1a6-8cadfb5508ac" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets_61314061-24e3-43fb-8e6d-c349d4985c70" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_f57440ca-7f33-4a1d-8de2-2512c65d6882" xlink:to="loc_us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets_61314061-24e3-43fb-8e6d-c349d4985c70" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInAccountsPayableAndAccruedLiabilities_37111599-5c8d-48c0-b134-f2763eba0611" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInAccountsPayableAndAccruedLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_f57440ca-7f33-4a1d-8de2-2512c65d6882" xlink:to="loc_us-gaap_IncreaseDecreaseInAccountsPayableAndAccruedLiabilities_37111599-5c8d-48c0-b134-f2763eba0611" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInOtherOperatingLiabilities_f9a4e999-f4bf-4b6d-8f76-48b7096bee6b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInOtherOperatingLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_f57440ca-7f33-4a1d-8de2-2512c65d6882" xlink:to="loc_us-gaap_IncreaseDecreaseInOtherOperatingLiabilities_f9a4e999-f4bf-4b6d-8f76-48b7096bee6b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_5cac5076-1b0a-40e9-ae6f-c09efb2cee0c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_15d468dc-3eef-46a5-b457-2244a03afa19" xlink:to="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_5cac5076-1b0a-40e9-ae6f-c09efb2cee0c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperationsAbstract_e76447b5-a364-4c9b-8360-62ccd7d0bc4a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperationsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_4dcb27ce-9e98-4179-b1e4-37c3efc97eb6" xlink:to="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperationsAbstract_e76447b5-a364-4c9b-8360-62ccd7d0bc4a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_eb013034-313f-45fa-8917-48c16a802117" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquirePropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperationsAbstract_e76447b5-a364-4c9b-8360-62ccd7d0bc4a" xlink:to="loc_us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_eb013034-313f-45fa-8917-48c16a802117" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToDevelopSoftware_ead08c57-69ca-4ce5-ab9a-48b160238475" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToDevelopSoftware"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperationsAbstract_e76447b5-a364-4c9b-8360-62ccd7d0bc4a" xlink:to="loc_us-gaap_PaymentsToDevelopSoftware_ead08c57-69ca-4ce5-ab9a-48b160238475" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired_124617e1-33cd-40fa-a237-f9573f8f2885" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperationsAbstract_e76447b5-a364-4c9b-8360-62ccd7d0bc4a" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired_124617e1-33cd-40fa-a237-f9573f8f2885" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsForProceedsFromOtherInvestingActivities_9ba581f5-7817-4fcb-8598-a76393986f56" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsForProceedsFromOtherInvestingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperationsAbstract_e76447b5-a364-4c9b-8360-62ccd7d0bc4a" xlink:to="loc_us-gaap_PaymentsForProceedsFromOtherInvestingActivities_9ba581f5-7817-4fcb-8598-a76393986f56" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_3647c610-7441-499d-99ba-e078a2241fae" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperationsAbstract_e76447b5-a364-4c9b-8360-62ccd7d0bc4a" xlink:to="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_3647c610-7441-499d-99ba-e078a2241fae" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract_a8aafa34-60a6-48a1-a404-e3e050120a7f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_4dcb27ce-9e98-4179-b1e4-37c3efc97eb6" xlink:to="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract_a8aafa34-60a6-48a1-a404-e3e050120a7f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfSecuredDebt_0a260e52-ccbb-4b99-8f2b-2ee09d074f5c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromIssuanceOfSecuredDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract_a8aafa34-60a6-48a1-a404-e3e050120a7f" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfSecuredDebt_0a260e52-ccbb-4b99-8f2b-2ee09d074f5c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RepaymentsOfSecuredDebt_f3ffff72-1cd3-4b96-9e87-77110dbb6801" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RepaymentsOfSecuredDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract_a8aafa34-60a6-48a1-a404-e3e050120a7f" xlink:to="loc_us-gaap_RepaymentsOfSecuredDebt_f3ffff72-1cd3-4b96-9e87-77110dbb6801" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsOfDebtIssuanceCosts_b48bb151-a26c-4cdd-b62c-991bf6a79ee2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsOfDebtIssuanceCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract_a8aafa34-60a6-48a1-a404-e3e050120a7f" xlink:to="loc_us-gaap_PaymentsOfDebtIssuanceCosts_b48bb151-a26c-4cdd-b62c-991bf6a79ee2" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RepaymentsOfLongTermDebt_637e1f54-87cc-47bc-8f18-d7c5437785c6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RepaymentsOfLongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract_a8aafa34-60a6-48a1-a404-e3e050120a7f" xlink:to="loc_us-gaap_RepaymentsOfLongTermDebt_637e1f54-87cc-47bc-8f18-d7c5437785c6" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RepaymentsOfConvertibleDebt_3461f722-c90f-4fe7-878f-0c7583bab196" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RepaymentsOfConvertibleDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract_a8aafa34-60a6-48a1-a404-e3e050120a7f" xlink:to="loc_us-gaap_RepaymentsOfConvertibleDebt_3461f722-c90f-4fe7-878f-0c7583bab196" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromStockOptionsExercised_74be63e1-8ce5-48d4-a1f0-d136f02c768f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract_a8aafa34-60a6-48a1-a404-e3e050120a7f" xlink:to="loc_us-gaap_ProceedsFromStockOptionsExercised_74be63e1-8ce5-48d4-a1f0-d136f02c768f" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfSharesUnderIncentiveAndShareBasedCompensationPlans_5c9a4669-8579-438e-a85e-df7b425ee250" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProceedsFromIssuanceOfSharesUnderIncentiveAndShareBasedCompensationPlans"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract_a8aafa34-60a6-48a1-a404-e3e050120a7f" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfSharesUnderIncentiveAndShareBasedCompensationPlans_5c9a4669-8579-438e-a85e-df7b425ee250" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsRelatedToTaxWithholdingForShareBasedCompensation_a7c9391e-bd0b-4e31-b59f-c031806cc535" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsRelatedToTaxWithholdingForShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract_a8aafa34-60a6-48a1-a404-e3e050120a7f" xlink:to="loc_us-gaap_PaymentsRelatedToTaxWithholdingForShareBasedCompensation_a7c9391e-bd0b-4e31-b59f-c031806cc535" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsForRepurchaseOfCommonStock_e2d8fdd3-67c4-461f-8c10-55560436fccb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsForRepurchaseOfCommonStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract_a8aafa34-60a6-48a1-a404-e3e050120a7f" xlink:to="loc_us-gaap_PaymentsForRepurchaseOfCommonStock_e2d8fdd3-67c4-461f-8c10-55560436fccb" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinanceLeasePrincipalPayments_630b4b91-02e7-4c7b-80e7-313fbc7aeffe" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinanceLeasePrincipalPayments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract_a8aafa34-60a6-48a1-a404-e3e050120a7f" xlink:to="loc_us-gaap_FinanceLeasePrincipalPayments_630b4b91-02e7-4c7b-80e7-313fbc7aeffe" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentForContingentConsiderationLiabilityFinancingActivities_460d25f1-74ef-4512-945d-155cedc6a2e1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentForContingentConsiderationLiabilityFinancingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract_a8aafa34-60a6-48a1-a404-e3e050120a7f" xlink:to="loc_us-gaap_PaymentForContingentConsiderationLiabilityFinancingActivities_460d25f1-74ef-4512-945d-155cedc6a2e1" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_52d492b5-75c1-4621-9625-315a3dcbb818" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract_a8aafa34-60a6-48a1-a404-e3e050120a7f" xlink:to="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_52d492b5-75c1-4621-9625-315a3dcbb818" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_4b5e9d1f-7a3d-45a6-b0af-78ff85635489" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_4dcb27ce-9e98-4179-b1e4-37c3efc97eb6" xlink:to="loc_us-gaap_EffectOfExchangeRateOnCashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_4b5e9d1f-7a3d-45a6-b0af-78ff85635489" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect_cdaa09e0-084a-4dcd-81b8-9b9642d2f53f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_4dcb27ce-9e98-4179-b1e4-37c3efc97eb6" xlink:to="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect_cdaa09e0-084a-4dcd-81b8-9b9642d2f53f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_90fe0a41-cbcd-41fd-b178-4e2e3ee5994f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_4dcb27ce-9e98-4179-b1e4-37c3efc97eb6" xlink:to="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_90fe0a41-cbcd-41fd-b178-4e2e3ee5994f" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents_15cd26bf-3fdb-4bb8-8783-ba7f2516b7a9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SupplementalCashFlowInformationAbstract_8030815b-11f8-4e2e-a64b-909e937c395e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SupplementalCashFlowInformationAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_4dcb27ce-9e98-4179-b1e4-37c3efc97eb6" xlink:to="loc_us-gaap_SupplementalCashFlowInformationAbstract_8030815b-11f8-4e2e-a64b-909e937c395e" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxesPaid_a0b150a4-7911-45ff-9a55-bd78a1eeb6a3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxesPaid"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplementalCashFlowInformationAbstract_8030815b-11f8-4e2e-a64b-909e937c395e" xlink:to="loc_us-gaap_IncomeTaxesPaid_a0b150a4-7911-45ff-9a55-bd78a1eeb6a3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestPaidNet_e2721421-9f7b-4093-9194-63462e296832" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestPaidNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplementalCashFlowInformationAbstract_8030815b-11f8-4e2e-a64b-909e937c395e" xlink:to="loc_us-gaap_InterestPaidNet_e2721421-9f7b-4093-9194-63462e296832" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalExpendituresIncurredButNotYetPaid_b05a9854-cade-416e-bcb4-fd35be00c80d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CapitalExpendituresIncurredButNotYetPaid"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplementalCashFlowInformationAbstract_8030815b-11f8-4e2e-a64b-909e937c395e" xlink:to="loc_us-gaap_CapitalExpendituresIncurredButNotYetPaid_b05a9854-cade-416e-bcb4-fd35be00c80d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount_b0ff3b1e-f09f-444f-83f1-657c66c939b8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplementalCashFlowInformationAbstract_8030815b-11f8-4e2e-a64b-909e937c395e" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount_b0ff3b1e-f09f-444f-83f1-657c66c939b8" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability_8bbec1e4-1dfb-4a55-a1a9-36a785768e4b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplementalCashFlowInformationAbstract_8030815b-11f8-4e2e-a64b-909e937c395e" xlink:to="loc_us-gaap_RightOfUseAssetObtainedInExchangeForOperatingLeaseLiability_8bbec1e4-1dfb-4a55-a1a9-36a785768e4b" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_OperatingLeaseRightOfUseAssetsReductionAndAdjustmentToOperatingLeaseLiabilitiesFromLeaseTerminations_6d4335a2-4e38-4799-b186-e9255b9646a7" xlink:href="mgni-20251231.xsd#mgni_OperatingLeaseRightOfUseAssetsReductionAndAdjustmentToOperatingLeaseLiabilitiesFromLeaseTerminations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplementalCashFlowInformationAbstract_8030815b-11f8-4e2e-a64b-909e937c395e" xlink:to="loc_mgni_OperatingLeaseRightOfUseAssetsReductionAndAdjustmentToOperatingLeaseLiabilitiesFromLeaseTerminations_6d4335a2-4e38-4799-b186-e9255b9646a7" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_BusinessCombinationConsiderationTransferredIndemnificationClaimsHoldback_3f206e09-19a5-482b-bc00-7248ca6f60d6" xlink:href="mgni-20251231.xsd#mgni_BusinessCombinationConsiderationTransferredIndemnificationClaimsHoldback"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplementalCashFlowInformationAbstract_8030815b-11f8-4e2e-a64b-909e937c395e" xlink:to="loc_mgni_BusinessCombinationConsiderationTransferredIndemnificationClaimsHoldback_3f206e09-19a5-482b-bc00-7248ca6f60d6" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_NoncashOrPartNoncashFinancingActivity_31742c90-ff0c-45a6-9e34-ec603787b5fe" xlink:href="mgni-20251231.xsd#mgni_NoncashOrPartNoncashFinancingActivity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplementalCashFlowInformationAbstract_8030815b-11f8-4e2e-a64b-909e937c395e" xlink:to="loc_mgni_NoncashOrPartNoncashFinancingActivity_31742c90-ff0c-45a6-9e34-ec603787b5fe" xlink:type="arc" order="8"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/OrganizationandSummaryofSignificantAccountingPoliciesNarrativeDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#OrganizationandSummaryofSignificantAccountingPoliciesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/OrganizationandSummaryofSignificantAccountingPoliciesNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskLineItems_e282f085-1d08-4ab8-825a-af44c88975aa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTable_f6aec84b-2669-46b0-a6bc-beaf300765e9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_e282f085-1d08-4ab8-825a-af44c88975aa" xlink:to="loc_us-gaap_ConcentrationRiskTable_f6aec84b-2669-46b0-a6bc-beaf300765e9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeAxis_405f371c-a8d7-4578-9189-fdee0279ddbf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_f6aec84b-2669-46b0-a6bc-beaf300765e9" xlink:to="loc_us-gaap_MeasurementInputTypeAxis_405f371c-a8d7-4578-9189-fdee0279ddbf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeDomain_405f371c-a8d7-4578-9189-fdee0279ddbf_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_MeasurementInputTypeAxis_405f371c-a8d7-4578-9189-fdee0279ddbf" xlink:to="loc_us-gaap_MeasurementInputTypeDomain_405f371c-a8d7-4578-9189-fdee0279ddbf_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeDomain_d530d6b5-ac1b-4078-9130-5f869352085a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_MeasurementInputTypeAxis_405f371c-a8d7-4578-9189-fdee0279ddbf" xlink:to="loc_us-gaap_MeasurementInputTypeDomain_d530d6b5-ac1b-4078-9130-5f869352085a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputExpectedDividendPaymentMember_8389b1f0-5211-4f96-8ccd-5438bdc40f96" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MeasurementInputExpectedDividendPaymentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_d530d6b5-ac1b-4078-9130-5f869352085a" xlink:to="loc_us-gaap_MeasurementInputExpectedDividendPaymentMember_8389b1f0-5211-4f96-8ccd-5438bdc40f96" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_fdbdddd0-d2ed-496d-a4ce-07aec120d24b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_f6aec84b-2669-46b0-a6bc-beaf300765e9" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_fdbdddd0-d2ed-496d-a4ce-07aec120d24b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_fdbdddd0-d2ed-496d-a4ce-07aec120d24b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_fdbdddd0-d2ed-496d-a4ce-07aec120d24b" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_fdbdddd0-d2ed-496d-a4ce-07aec120d24b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_9470bebb-3e4a-4072-a738-2be159135ac8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_fdbdddd0-d2ed-496d-a4ce-07aec120d24b" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_9470bebb-3e4a-4072-a738-2be159135ac8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ComputerSoftwareIntangibleAssetMember_e5810bd7-083f-4a27-ac17-b09d5c53bda2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ComputerSoftwareIntangibleAssetMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_9470bebb-3e4a-4072-a738-2be159135ac8" xlink:to="loc_us-gaap_ComputerSoftwareIntangibleAssetMember_e5810bd7-083f-4a27-ac17-b09d5c53bda2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByTypeAxis_59c43bc6-f4bd-4359-8e05-be07fe89dc51" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_f6aec84b-2669-46b0-a6bc-beaf300765e9" xlink:to="loc_us-gaap_ConcentrationRiskByTypeAxis_59c43bc6-f4bd-4359-8e05-be07fe89dc51" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_59c43bc6-f4bd-4359-8e05-be07fe89dc51_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_59c43bc6-f4bd-4359-8e05-be07fe89dc51" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_59c43bc6-f4bd-4359-8e05-be07fe89dc51_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_27e3d450-ef73-4774-9ad4-d05e99582aba" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_59c43bc6-f4bd-4359-8e05-be07fe89dc51" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_27e3d450-ef73-4774-9ad4-d05e99582aba" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerConcentrationRiskMember_9718272b-17d4-4d82-94d9-5ef8e8f0b54d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CustomerConcentrationRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_27e3d450-ef73-4774-9ad4-d05e99582aba" xlink:to="loc_us-gaap_CustomerConcentrationRiskMember_9718272b-17d4-4d82-94d9-5ef8e8f0b54d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SupplierConcentrationRiskMember_0cd5ef7c-bde1-42e7-9929-f6cacc375cc6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SupplierConcentrationRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_27e3d450-ef73-4774-9ad4-d05e99582aba" xlink:to="loc_us-gaap_SupplierConcentrationRiskMember_0cd5ef7c-bde1-42e7-9929-f6cacc375cc6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_fbe0b59d-0ecd-475a-8b26-ad87717db228" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_f6aec84b-2669-46b0-a6bc-beaf300765e9" xlink:to="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_fbe0b59d-0ecd-475a-8b26-ad87717db228" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_fbe0b59d-0ecd-475a-8b26-ad87717db228_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_fbe0b59d-0ecd-475a-8b26-ad87717db228" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_fbe0b59d-0ecd-475a-8b26-ad87717db228_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_3bf8bb82-8d5d-48e3-b894-2c7c74724150" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_fbe0b59d-0ecd-475a-8b26-ad87717db228" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_3bf8bb82-8d5d-48e3-b894-2c7c74724150" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableMember_2b74ef5f-4825-4999-93bd-87e3a9c27db2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountsReceivableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_3bf8bb82-8d5d-48e3-b894-2c7c74724150" xlink:to="loc_us-gaap_AccountsReceivableMember_2b74ef5f-4825-4999-93bd-87e3a9c27db2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsPayableMember_33384863-3705-436d-88a4-432b12cc6433" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountsPayableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_3bf8bb82-8d5d-48e3-b894-2c7c74724150" xlink:to="loc_us-gaap_AccountsPayableMember_33384863-3705-436d-88a4-432b12cc6433" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MajorCustomersAxis_74e56522-d136-4ee3-a875-f6e13595fce6" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MajorCustomersAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_f6aec84b-2669-46b0-a6bc-beaf300765e9" xlink:to="loc_srt_MajorCustomersAxis_74e56522-d136-4ee3-a875-f6e13595fce6" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NameOfMajorCustomerDomain_74e56522-d136-4ee3-a875-f6e13595fce6_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_NameOfMajorCustomerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_MajorCustomersAxis_74e56522-d136-4ee3-a875-f6e13595fce6" xlink:to="loc_srt_NameOfMajorCustomerDomain_74e56522-d136-4ee3-a875-f6e13595fce6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NameOfMajorCustomerDomain_38415b91-4b8e-4074-969b-0897bf58a877" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_NameOfMajorCustomerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_MajorCustomersAxis_74e56522-d136-4ee3-a875-f6e13595fce6" xlink:to="loc_srt_NameOfMajorCustomerDomain_38415b91-4b8e-4074-969b-0897bf58a877" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_CustomerOneMember_893c1a6d-ac84-4649-84f5-bc9fb40171ee" xlink:href="mgni-20251231.xsd#mgni_CustomerOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_38415b91-4b8e-4074-969b-0897bf58a877" xlink:to="loc_mgni_CustomerOneMember_893c1a6d-ac84-4649-84f5-bc9fb40171ee" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_CustomerTwoMember_f6386dd7-4045-4661-b0e5-0c68a5343869" xlink:href="mgni-20251231.xsd#mgni_CustomerTwoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_38415b91-4b8e-4074-969b-0897bf58a877" xlink:to="loc_mgni_CustomerTwoMember_f6386dd7-4045-4661-b0e5-0c68a5343869" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ChangeInAccountingEstimateByTypeAxis_019c7089-d21d-76d3-80be-f1b8f7adee51" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ChangeInAccountingEstimateByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_f6aec84b-2669-46b0-a6bc-beaf300765e9" xlink:to="loc_us-gaap_ChangeInAccountingEstimateByTypeAxis_019c7089-d21d-76d3-80be-f1b8f7adee51" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ChangeInAccountingEstimateTypeDomain_019c7089-d21d-76d3-80be-f1b8f7adee51_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ChangeInAccountingEstimateTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ChangeInAccountingEstimateByTypeAxis_019c7089-d21d-76d3-80be-f1b8f7adee51" xlink:to="loc_us-gaap_ChangeInAccountingEstimateTypeDomain_019c7089-d21d-76d3-80be-f1b8f7adee51_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ChangeInAccountingEstimateTypeDomain_019c708a-1383-7821-8ac4-8fa185b217a2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ChangeInAccountingEstimateTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ChangeInAccountingEstimateByTypeAxis_019c7089-d21d-76d3-80be-f1b8f7adee51" xlink:to="loc_us-gaap_ChangeInAccountingEstimateTypeDomain_019c708a-1383-7821-8ac4-8fa185b217a2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ChangeInAccountingMethodAccountedForAsChangeInEstimateMember_019c708a-6d78-7ea9-bee3-077f35c5698a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ChangeInAccountingMethodAccountedForAsChangeInEstimateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ChangeInAccountingEstimateTypeDomain_019c708a-1383-7821-8ac4-8fa185b217a2" xlink:to="loc_us-gaap_ChangeInAccountingMethodAccountedForAsChangeInEstimateMember_019c708a-6d78-7ea9-bee3-077f35c5698a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis_1903a853-9efe-4158-ab4e-6cb31eff8f8d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_f6aec84b-2669-46b0-a6bc-beaf300765e9" xlink:to="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis_1903a853-9efe-4158-ab4e-6cb31eff8f8d" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain_1903a853-9efe-4158-ab4e-6cb31eff8f8d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis_1903a853-9efe-4158-ab4e-6cb31eff8f8d" xlink:to="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain_1903a853-9efe-4158-ab4e-6cb31eff8f8d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain_521d8467-5cb0-4b2f-934c-fdb260fca443" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionBySupplierAxis_1903a853-9efe-4158-ab4e-6cb31eff8f8d" xlink:to="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain_521d8467-5cb0-4b2f-934c-fdb260fca443" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_OneSellerMember_aa3e045a-d379-431f-aa45-c0f7c54161dc" xlink:href="mgni-20251231.xsd#mgni_OneSellerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedGoodsAndNonemployeeServicesTransactionSupplierDomain_521d8467-5cb0-4b2f-934c-fdb260fca443" xlink:to="loc_mgni_OneSellerMember_aa3e045a-d379-431f-aa45-c0f7c54161dc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_08d348ef-6345-48b3-a0f8-87b2b49e1059" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_f6aec84b-2669-46b0-a6bc-beaf300765e9" xlink:to="loc_us-gaap_AwardTypeAxis_08d348ef-6345-48b3-a0f8-87b2b49e1059" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_08d348ef-6345-48b3-a0f8-87b2b49e1059_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_08d348ef-6345-48b3-a0f8-87b2b49e1059" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_08d348ef-6345-48b3-a0f8-87b2b49e1059_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_6eb3fbfc-8013-4c59-91ca-e8a30d462da8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_08d348ef-6345-48b3-a0f8-87b2b49e1059" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_6eb3fbfc-8013-4c59-91ca-e8a30d462da8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_b4d27cf7-57a5-4863-a368-eb89ca858a2b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_6eb3fbfc-8013-4c59-91ca-e8a30d462da8" xlink:to="loc_us-gaap_EmployeeStockOptionMember_b4d27cf7-57a5-4863-a368-eb89ca858a2b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfOperatingSegments_2de2d1d1-4bae-4189-a799-e7333fa520a8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NumberOfOperatingSegments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_e282f085-1d08-4ab8-825a-af44c88975aa" xlink:to="loc_us-gaap_NumberOfOperatingSegments_2de2d1d1-4bae-4189-a799-e7333fa520a8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfReportableSegments_e06fec41-ed14-4a6f-9472-c27fe43db4c8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NumberOfReportableSegments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_e282f085-1d08-4ab8-825a-af44c88975aa" xlink:to="loc_us-gaap_NumberOfReportableSegments_e06fec41-ed14-4a6f-9472-c27fe43db4c8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentUsefulLife_019c708c-200f-7496-848a-dcae13658a71" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_e282f085-1d08-4ab8-825a-af44c88975aa" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentUsefulLife_019c708c-200f-7496-848a-dcae13658a71" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingIncomeLoss_019c708e-86c2-758d-bc68-e8e3d6f9d3bf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_e282f085-1d08-4ab8-825a-af44c88975aa" xlink:to="loc_us-gaap_OperatingIncomeLoss_019c708e-86c2-758d-bc68-e8e3d6f9d3bf" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019c708f-fe62-7cee-80ae-4708640d0c0a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_e282f085-1d08-4ab8-825a-af44c88975aa" xlink:to="loc_us-gaap_NetIncomeLoss_019c708f-fe62-7cee-80ae-4708640d0c0a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareBasic_019c7090-ed99-72ff-87b8-0fd804e1b052" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_e282f085-1d08-4ab8-825a-af44c88975aa" xlink:to="loc_us-gaap_EarningsPerShareBasic_019c7090-ed99-72ff-87b8-0fd804e1b052" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareDiluted_019c7093-2e2f-7c59-99d7-90024696c5f1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareDiluted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_e282f085-1d08-4ab8-825a-af44c88975aa" xlink:to="loc_us-gaap_EarningsPerShareDiluted_019c7093-2e2f-7c59-99d7-90024696c5f1" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardTermsOfAward_d3a0d370-252b-4f89-bfb0-7c45a3f05734" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardTermsOfAward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_e282f085-1d08-4ab8-825a-af44c88975aa" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardTermsOfAward_d3a0d370-252b-4f89-bfb0-7c45a3f05734" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1_bddad048-79e1-4b0b-95a5-8258ac9b7f90" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_e282f085-1d08-4ab8-825a-af44c88975aa" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1_bddad048-79e1-4b0b-95a5-8258ac9b7f90" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AlternativeInvestmentMeasurementInput_28bbd494-3e2b-440f-8cbf-def40a045305" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AlternativeInvestmentMeasurementInput"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_e282f085-1d08-4ab8-825a-af44c88975aa" xlink:to="loc_us-gaap_AlternativeInvestmentMeasurementInput_28bbd494-3e2b-440f-8cbf-def40a045305" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_4785c4c2-7f66-42fb-b452-ab2ebf7e59a0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_e282f085-1d08-4ab8-825a-af44c88975aa" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_4785c4c2-7f66-42fb-b452-ab2ebf7e59a0" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesAuthorized_5fa64d44-ea80-4f72-8be7-e3f7151e2389" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_e282f085-1d08-4ab8-825a-af44c88975aa" xlink:to="loc_us-gaap_PreferredStockSharesAuthorized_5fa64d44-ea80-4f72-8be7-e3f7151e2389" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_e199e472-13be-4f51-9f2e-fe4cdce3df82" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockCapitalSharesReservedForFutureIssuance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_e282f085-1d08-4ab8-825a-af44c88975aa" xlink:to="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_e199e472-13be-4f51-9f2e-fe4cdce3df82" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_FiniteLivedIntangibleAssetAmortizationTerm_00ff88cf-0ecb-4af5-b135-5ce3599411ca" xlink:href="mgni-20251231.xsd#mgni_FiniteLivedIntangibleAssetAmortizationTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_e282f085-1d08-4ab8-825a-af44c88975aa" xlink:to="loc_mgni_FiniteLivedIntangibleAssetAmortizationTerm_00ff88cf-0ecb-4af5-b135-5ce3599411ca" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfReportingUnits_31317924-5bb4-4b00-9ce4-6100375bbbc4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NumberOfReportingUnits"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_e282f085-1d08-4ab8-825a-af44c88975aa" xlink:to="loc_us-gaap_NumberOfReportingUnits_31317924-5bb4-4b00-9ce4-6100375bbbc4" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskPercentage1_e9156050-1a36-4bb0-a3a6-be81ca295e21" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskPercentage1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_e282f085-1d08-4ab8-825a-af44c88975aa" xlink:to="loc_us-gaap_ConcentrationRiskPercentage1_e9156050-1a36-4bb0-a3a6-be81ca295e21" xlink:type="arc" order="15"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/OrganizationandSummaryofSignificantAccountingPoliciesPropertyandEquipmentDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#OrganizationandSummaryofSignificantAccountingPoliciesPropertyandEquipmentDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/OrganizationandSummaryofSignificantAccountingPoliciesPropertyandEquipmentDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_265bb544-e9f5-40e4-bf7b-94ae4666446c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_b12f0f16-d72e-4ec0-b1d9-f7ba6aaf4617" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_265bb544-e9f5-40e4-bf7b-94ae4666446c" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_b12f0f16-d72e-4ec0-b1d9-f7ba6aaf4617" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_30414bbd-5b25-43e6-9fb1-681c8354cad0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_b12f0f16-d72e-4ec0-b1d9-f7ba6aaf4617" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_30414bbd-5b25-43e6-9fb1-681c8354cad0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_30414bbd-5b25-43e6-9fb1-681c8354cad0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_30414bbd-5b25-43e6-9fb1-681c8354cad0" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_30414bbd-5b25-43e6-9fb1-681c8354cad0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_5c98f1ac-3421-48a5-a655-1b438f93f76c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_30414bbd-5b25-43e6-9fb1-681c8354cad0" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_5c98f1ac-3421-48a5-a655-1b438f93f76c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ComputerEquipmentMember_da86d996-ad38-4f4c-a016-d9d0fa392f9d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ComputerEquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_5c98f1ac-3421-48a5-a655-1b438f93f76c" xlink:to="loc_us-gaap_ComputerEquipmentMember_da86d996-ad38-4f4c-a016-d9d0fa392f9d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FurnitureAndFixturesMember_b25f8377-c77f-4720-ba83-5980f7cb0347" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FurnitureAndFixturesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_5c98f1ac-3421-48a5-a655-1b438f93f76c" xlink:to="loc_us-gaap_FurnitureAndFixturesMember_b25f8377-c77f-4720-ba83-5980f7cb0347" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SoftwareAndSoftwareDevelopmentCostsMember_95d16795-68c4-46be-ba80-d7a9c57677b8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SoftwareAndSoftwareDevelopmentCostsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_5c98f1ac-3421-48a5-a655-1b438f93f76c" xlink:to="loc_us-gaap_SoftwareAndSoftwareDevelopmentCostsMember_95d16795-68c4-46be-ba80-d7a9c57677b8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_348ed19c-af52-4e61-8d0e-f59d5f3997a0" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_b12f0f16-d72e-4ec0-b1d9-f7ba6aaf4617" xlink:to="loc_srt_RangeAxis_348ed19c-af52-4e61-8d0e-f59d5f3997a0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_348ed19c-af52-4e61-8d0e-f59d5f3997a0_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_348ed19c-af52-4e61-8d0e-f59d5f3997a0" xlink:to="loc_srt_RangeMember_348ed19c-af52-4e61-8d0e-f59d5f3997a0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_a28483c5-d489-45df-86ff-070ae3b7f997" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_348ed19c-af52-4e61-8d0e-f59d5f3997a0" xlink:to="loc_srt_RangeMember_a28483c5-d489-45df-86ff-070ae3b7f997" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_f6624510-0834-4894-baf7-86f182ef5997" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_a28483c5-d489-45df-86ff-070ae3b7f997" xlink:to="loc_srt_MinimumMember_f6624510-0834-4894-baf7-86f182ef5997" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_cf16946e-6d3b-4734-a8c7-81271a4dc76d" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_a28483c5-d489-45df-86ff-070ae3b7f997" xlink:to="loc_srt_MaximumMember_cf16946e-6d3b-4734-a8c7-81271a4dc76d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentUsefulLife_1c077e28-0842-4adb-9fae-2aeda37333da" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_265bb544-e9f5-40e4-bf7b-94ae4666446c" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentUsefulLife_1c077e28-0842-4adb-9fae-2aeda37333da" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/OrganizationandSummaryofSignificantAccountingPoliciesIntangibleAssetsDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#OrganizationandSummaryofSignificantAccountingPoliciesIntangibleAssetsDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/OrganizationandSummaryofSignificantAccountingPoliciesIntangibleAssetsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_ddd9eda5-b1d6-41d5-9091-423c735c1a89" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_cb1a6418-0793-41ec-9a87-2591aaa4aea4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_ddd9eda5-b1d6-41d5-9091-423c735c1a89" xlink:to="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_cb1a6418-0793-41ec-9a87-2591aaa4aea4" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_74cd006b-f7a4-4aaf-8475-1b8fcdd52b95" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_cb1a6418-0793-41ec-9a87-2591aaa4aea4" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_74cd006b-f7a4-4aaf-8475-1b8fcdd52b95" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_74cd006b-f7a4-4aaf-8475-1b8fcdd52b95_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_74cd006b-f7a4-4aaf-8475-1b8fcdd52b95" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_74cd006b-f7a4-4aaf-8475-1b8fcdd52b95_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_2e12855a-3670-49d1-a907-6d2ebeecaded" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_74cd006b-f7a4-4aaf-8475-1b8fcdd52b95" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_2e12855a-3670-49d1-a907-6d2ebeecaded" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DevelopedTechnologyRightsMember_762c84e3-fc7e-4dcb-847a-27e3bdda73ee" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DevelopedTechnologyRightsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_2e12855a-3670-49d1-a907-6d2ebeecaded" xlink:to="loc_us-gaap_DevelopedTechnologyRightsMember_762c84e3-fc7e-4dcb-847a-27e3bdda73ee" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InProcessResearchAndDevelopmentMember_c6e1dfbf-8e4e-4b78-847c-4e723ea6432c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InProcessResearchAndDevelopmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_2e12855a-3670-49d1-a907-6d2ebeecaded" xlink:to="loc_us-gaap_InProcessResearchAndDevelopmentMember_c6e1dfbf-8e4e-4b78-847c-4e723ea6432c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_df059ef0-3979-426c-88ea-2f5a9181f43e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_2e12855a-3670-49d1-a907-6d2ebeecaded" xlink:to="loc_us-gaap_CustomerRelationshipsMember_df059ef0-3979-426c-88ea-2f5a9181f43e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncompeteAgreementsMember_8cec5351-ae8c-45c5-9729-6ac7295955ad" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NoncompeteAgreementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_2e12855a-3670-49d1-a907-6d2ebeecaded" xlink:to="loc_us-gaap_NoncompeteAgreementsMember_8cec5351-ae8c-45c5-9729-6ac7295955ad" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherIntangibleAssetsMember_d9bd2f30-5f06-43c9-a67c-b996f1c00e69" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherIntangibleAssetsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_2e12855a-3670-49d1-a907-6d2ebeecaded" xlink:to="loc_us-gaap_OtherIntangibleAssetsMember_d9bd2f30-5f06-43c9-a67c-b996f1c00e69" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_7409929b-2874-4d0c-a106-83482bfbcb07" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_cb1a6418-0793-41ec-9a87-2591aaa4aea4" xlink:to="loc_srt_RangeAxis_7409929b-2874-4d0c-a106-83482bfbcb07" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_7409929b-2874-4d0c-a106-83482bfbcb07_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_7409929b-2874-4d0c-a106-83482bfbcb07" xlink:to="loc_srt_RangeMember_7409929b-2874-4d0c-a106-83482bfbcb07_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_ea691c7e-a67d-4eee-a8c3-981dc8355782" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_7409929b-2874-4d0c-a106-83482bfbcb07" xlink:to="loc_srt_RangeMember_ea691c7e-a67d-4eee-a8c3-981dc8355782" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_7c8b0edb-d59d-4916-9232-fb0fa9c9085c" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_ea691c7e-a67d-4eee-a8c3-981dc8355782" xlink:to="loc_srt_MinimumMember_7c8b0edb-d59d-4916-9232-fb0fa9c9085c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_12917538-6b77-4903-bff1-213a6524a006" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_ea691c7e-a67d-4eee-a8c3-981dc8355782" xlink:to="loc_srt_MaximumMember_12917538-6b77-4903-bff1-213a6524a006" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_13e595fb-6d8b-4a5c-ac70-060f5e55758a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_ddd9eda5-b1d6-41d5-9091-423c735c1a89" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_13e595fb-6d8b-4a5c-ac70-060f5e55758a" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/NetIncomeLossPerShareBasicandDilutedIncomeLossPerShareDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#NetIncomeLossPerShareBasicandDilutedIncomeLossPerShareDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/NetIncomeLossPerShareBasicandDilutedIncomeLossPerShareDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_50d18e6c-269e-490d-a862-1a4be5e5f643" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_ab24ef75-8319-4553-aa8c-d9179c5edead" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_50d18e6c-269e-490d-a862-1a4be5e5f643" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_ab24ef75-8319-4553-aa8c-d9179c5edead" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_e0c549f6-2997-4ee5-a12a-23ea99bcbc91" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_ab24ef75-8319-4553-aa8c-d9179c5edead" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_e0c549f6-2997-4ee5-a12a-23ea99bcbc91" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_e0c549f6-2997-4ee5-a12a-23ea99bcbc91_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_e0c549f6-2997-4ee5-a12a-23ea99bcbc91" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_e0c549f6-2997-4ee5-a12a-23ea99bcbc91_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_c98fc99c-5697-4b0f-b35a-36c8a7426b84" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_e0c549f6-2997-4ee5-a12a-23ea99bcbc91" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_c98fc99c-5697-4b0f-b35a-36c8a7426b84" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_b7c0df3a-4b87-4bc8-8f52-5f1e2189726b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_c98fc99c-5697-4b0f-b35a-36c8a7426b84" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_b7c0df3a-4b87-4bc8-8f52-5f1e2189726b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_bee9e06f-c360-42cc-8b01-8bcc41626dfa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_c98fc99c-5697-4b0f-b35a-36c8a7426b84" xlink:to="loc_us-gaap_EmployeeStockOptionMember_bee9e06f-c360-42cc-8b01-8bcc41626dfa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_03d299b0-299b-4d1e-af9a-b6ff600219c9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_c98fc99c-5697-4b0f-b35a-36c8a7426b84" xlink:to="loc_us-gaap_PerformanceSharesMember_03d299b0-299b-4d1e-af9a-b6ff600219c9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_2d656b46-69a7-49bc-b768-5563ba122833" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_c98fc99c-5697-4b0f-b35a-36c8a7426b84" xlink:to="loc_us-gaap_EmployeeStockMember_2d656b46-69a7-49bc-b768-5563ba122833" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareBasicAbstract_1131fa7f-67cc-4bf4-b7fc-23c7024b34e7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareBasicAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_50d18e6c-269e-490d-a862-1a4be5e5f643" xlink:to="loc_us-gaap_EarningsPerShareBasicAbstract_1131fa7f-67cc-4bf4-b7fc-23c7024b34e7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_77d085ef-23b3-458a-ba71-8e1ea8ef9a35" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareBasicAbstract_1131fa7f-67cc-4bf4-b7fc-23c7024b34e7" xlink:to="loc_us-gaap_NetIncomeLoss_77d085ef-23b3-458a-ba71-8e1ea8ef9a35" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_788ed821-c5a9-4cb4-9381-408de70cae17" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareBasicAbstract_1131fa7f-67cc-4bf4-b7fc-23c7024b34e7" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_788ed821-c5a9-4cb4-9381-408de70cae17" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareBasic_053bd4ac-c2a7-4d08-80e1-19c9c943c42f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareBasicAbstract_1131fa7f-67cc-4bf4-b7fc-23c7024b34e7" xlink:to="loc_us-gaap_EarningsPerShareBasic_053bd4ac-c2a7-4d08-80e1-19c9c943c42f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareDilutedAbstract_7c80c6b8-e074-4fa8-b2a2-9a6f25041b66" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareDilutedAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_50d18e6c-269e-490d-a862-1a4be5e5f643" xlink:to="loc_us-gaap_EarningsPerShareDilutedAbstract_7c80c6b8-e074-4fa8-b2a2-9a6f25041b66" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestOnConvertibleDebtNetOfTax_019c4c2d-8d14-7af9-8047-658dfa629a80" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestOnConvertibleDebtNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareDilutedAbstract_7c80c6b8-e074-4fa8-b2a2-9a6f25041b66" xlink:to="loc_us-gaap_InterestOnConvertibleDebtNetOfTax_019c4c2d-8d14-7af9-8047-658dfa629a80" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersDiluted_6062d138-21b0-42d1-8743-5bd75433b3d4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLossAvailableToCommonStockholdersDiluted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareDilutedAbstract_7c80c6b8-e074-4fa8-b2a2-9a6f25041b66" xlink:to="loc_us-gaap_NetIncomeLossAvailableToCommonStockholdersDiluted_6062d138-21b0-42d1-8743-5bd75433b3d4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_8dd09248-a0bc-4a5a-af60-cb2ac4021729" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_50d18e6c-269e-490d-a862-1a4be5e5f643" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_8dd09248-a0bc-4a5a-af60-cb2ac4021729" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements_4c712353-d33f-4d1f-b192-1c987be08e6a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_8dd09248-a0bc-4a5a-af60-cb2ac4021729" xlink:to="loc_us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements_4c712353-d33f-4d1f-b192-1c987be08e6a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncrementalCommonSharesAttributableToConversionOfDebtSecurities_019c4c31-4a95-76b9-9008-0dd17f693435" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncrementalCommonSharesAttributableToConversionOfDebtSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_8dd09248-a0bc-4a5a-af60-cb2ac4021729" xlink:to="loc_us-gaap_IncrementalCommonSharesAttributableToConversionOfDebtSecurities_019c4c31-4a95-76b9-9008-0dd17f693435" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_cf5d20ed-b339-4c9b-8232-4d59a1f75f5d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_8dd09248-a0bc-4a5a-af60-cb2ac4021729" xlink:to="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_cf5d20ed-b339-4c9b-8232-4d59a1f75f5d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareDiluted_56dc5309-4bce-4bc8-bd71-a41c58b74876" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareDiluted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_8dd09248-a0bc-4a5a-af60-cb2ac4021729" xlink:to="loc_us-gaap_EarningsPerShareDiluted_56dc5309-4bce-4bc8-bd71-a41c58b74876" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/NetIncomeLossPerShareSharesExcludedandIncludedinCalculationofDilutedEarningsPerShareDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#NetIncomeLossPerShareSharesExcludedandIncludedinCalculationofDilutedEarningsPerShareDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/NetIncomeLossPerShareSharesExcludedandIncludedinCalculationofDilutedEarningsPerShareDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_8c277fe2-ae71-4e2a-8438-7ddfb8337885" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_bec0b096-2195-4c71-b9c1-3a5abcbc44f4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_8c277fe2-ae71-4e2a-8438-7ddfb8337885" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_bec0b096-2195-4c71-b9c1-3a5abcbc44f4" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_d6504843-e939-47ad-9197-3e51fc4378be" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_bec0b096-2195-4c71-b9c1-3a5abcbc44f4" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_d6504843-e939-47ad-9197-3e51fc4378be" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_d6504843-e939-47ad-9197-3e51fc4378be_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_d6504843-e939-47ad-9197-3e51fc4378be" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_d6504843-e939-47ad-9197-3e51fc4378be_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_c71354fb-496f-478e-9c3d-dc8de52bd4d3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_d6504843-e939-47ad-9197-3e51fc4378be" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_c71354fb-496f-478e-9c3d-dc8de52bd4d3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_4b1f7829-ca6a-4f3b-a956-896ef0ab6d4c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_c71354fb-496f-478e-9c3d-dc8de52bd4d3" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_4b1f7829-ca6a-4f3b-a956-896ef0ab6d4c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_fd6e4ff7-4cd6-4a1a-8487-2c8ec9696a14" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_c71354fb-496f-478e-9c3d-dc8de52bd4d3" xlink:to="loc_us-gaap_EmployeeStockOptionMember_fd6e4ff7-4cd6-4a1a-8487-2c8ec9696a14" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_a18990fc-e052-4779-b0de-e41aa2f08064" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_c71354fb-496f-478e-9c3d-dc8de52bd4d3" xlink:to="loc_us-gaap_PerformanceSharesMember_a18990fc-e052-4779-b0de-e41aa2f08064" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_afdf2253-7ec6-497e-a52c-e3c5ef070bd3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_c71354fb-496f-478e-9c3d-dc8de52bd4d3" xlink:to="loc_us-gaap_EmployeeStockMember_afdf2253-7ec6-497e-a52c-e3c5ef070bd3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtSecuritiesMember_d1d24632-efe9-4add-a735-edf8a084be28" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConvertibleDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_c71354fb-496f-478e-9c3d-dc8de52bd4d3" xlink:to="loc_us-gaap_ConvertibleDebtSecuritiesMember_d1d24632-efe9-4add-a735-edf8a084be28" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_e0003226-ea55-49bb-9db4-4c80c6900a93" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_8c277fe2-ae71-4e2a-8438-7ddfb8337885" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_e0003226-ea55-49bb-9db4-4c80c6900a93" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/NetIncomeLossPerShareNarrativeDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#NetIncomeLossPerShareNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/NetIncomeLossPerShareNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_a9faf068-75a3-4cd8-8d29-23e976d42291" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_afa7d807-6a02-4fc7-ba6d-c228732555e0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_a9faf068-75a3-4cd8-8d29-23e976d42291" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_afa7d807-6a02-4fc7-ba6d-c228732555e0" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_9dc3e3d7-0773-4e4b-96ba-9ba862b9f6df" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_afa7d807-6a02-4fc7-ba6d-c228732555e0" xlink:to="loc_us-gaap_AwardTypeAxis_9dc3e3d7-0773-4e4b-96ba-9ba862b9f6df" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_9dc3e3d7-0773-4e4b-96ba-9ba862b9f6df_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_9dc3e3d7-0773-4e4b-96ba-9ba862b9f6df" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_9dc3e3d7-0773-4e4b-96ba-9ba862b9f6df_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_037fc1a5-bcee-4c2b-b6ef-cf483c010b58" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_9dc3e3d7-0773-4e4b-96ba-9ba862b9f6df" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_037fc1a5-bcee-4c2b-b6ef-cf483c010b58" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_PerformanceSharesGrantedIn2021Member_160344de-d61d-4ccb-a6bb-62daf2c00a3f" xlink:href="mgni-20251231.xsd#mgni_PerformanceSharesGrantedIn2021Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_037fc1a5-bcee-4c2b-b6ef-cf483c010b58" xlink:to="loc_mgni_PerformanceSharesGrantedIn2021Member_160344de-d61d-4ccb-a6bb-62daf2c00a3f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_PerformanceSharesGrantedIn2022Member_ada965bb-a451-47f1-92d9-c5247beae825" xlink:href="mgni-20251231.xsd#mgni_PerformanceSharesGrantedIn2022Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_037fc1a5-bcee-4c2b-b6ef-cf483c010b58" xlink:to="loc_mgni_PerformanceSharesGrantedIn2022Member_ada965bb-a451-47f1-92d9-c5247beae825" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_PerformanceSharesGrantedIn2023Member_04dcb095-c284-4091-98f4-a0109a02e8cb" xlink:href="mgni-20251231.xsd#mgni_PerformanceSharesGrantedIn2023Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_037fc1a5-bcee-4c2b-b6ef-cf483c010b58" xlink:to="loc_mgni_PerformanceSharesGrantedIn2023Member_04dcb095-c284-4091-98f4-a0109a02e8cb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_PerformanceSharesGrantedIn2024Member_a83c1ad0-29ff-4673-88ae-da025b655e31" xlink:href="mgni-20251231.xsd#mgni_PerformanceSharesGrantedIn2024Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_037fc1a5-bcee-4c2b-b6ef-cf483c010b58" xlink:to="loc_mgni_PerformanceSharesGrantedIn2024Member_a83c1ad0-29ff-4673-88ae-da025b655e31" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_PerformanceSharesGrantedIn2025Member_af23ccf4-3eb0-4430-9578-25a8ebe44183" xlink:href="mgni-20251231.xsd#mgni_PerformanceSharesGrantedIn2025Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_037fc1a5-bcee-4c2b-b6ef-cf483c010b58" xlink:to="loc_mgni_PerformanceSharesGrantedIn2025Member_af23ccf4-3eb0-4430-9578-25a8ebe44183" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_f0f66538-1618-4fe0-9241-4cb3c01e5eff" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_afa7d807-6a02-4fc7-ba6d-c228732555e0" xlink:to="loc_us-gaap_DebtInstrumentAxis_f0f66538-1618-4fe0-9241-4cb3c01e5eff" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_f0f66538-1618-4fe0-9241-4cb3c01e5eff_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_f0f66538-1618-4fe0-9241-4cb3c01e5eff" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_f0f66538-1618-4fe0-9241-4cb3c01e5eff_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_428e330a-4a7b-468b-9dc9-49287292388c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_f0f66538-1618-4fe0-9241-4cb3c01e5eff" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_428e330a-4a7b-468b-9dc9-49287292388c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ConvertibleSeniorNoteMember_ca446e9b-1b4e-41ff-a710-d9cd4cd88f86" xlink:href="mgni-20251231.xsd#mgni_ConvertibleSeniorNoteMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_428e330a-4a7b-468b-9dc9-49287292388c" xlink:to="loc_mgni_ConvertibleSeniorNoteMember_ca446e9b-1b4e-41ff-a710-d9cd4cd88f86" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_cb0bd1ef-5f2a-4d8f-a403-77869dd06f0c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_afa7d807-6a02-4fc7-ba6d-c228732555e0" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_cb0bd1ef-5f2a-4d8f-a403-77869dd06f0c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_cb0bd1ef-5f2a-4d8f-a403-77869dd06f0c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_cb0bd1ef-5f2a-4d8f-a403-77869dd06f0c" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_cb0bd1ef-5f2a-4d8f-a403-77869dd06f0c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_2069c4e1-96ae-4691-acc2-55c47e0598a8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_cb0bd1ef-5f2a-4d8f-a403-77869dd06f0c" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_2069c4e1-96ae-4691-acc2-55c47e0598a8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtMember_4c14fc72-3737-4f47-bdb8-9fbb72a0354a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConvertibleDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_2069c4e1-96ae-4691-acc2-55c47e0598a8" xlink:to="loc_us-gaap_ConvertibleDebtMember_4c14fc72-3737-4f47-bdb8-9fbb72a0354a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardPerformanceMeasurementPercentage_266c721e-6e58-434e-ba1e-7e933d515c9a" xlink:href="mgni-20251231.xsd#mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardPerformanceMeasurementPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_a9faf068-75a3-4cd8-8d29-23e976d42291" xlink:to="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardPerformanceMeasurementPercentage_266c721e-6e58-434e-ba1e-7e933d515c9a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleConversionRatio1_00213e80-8b69-431b-8e5b-bdd0c14e1f0a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentConvertibleConversionRatio1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_a9faf068-75a3-4cd8-8d29-23e976d42291" xlink:to="loc_us-gaap_DebtInstrumentConvertibleConversionRatio1_00213e80-8b69-431b-8e5b-bdd0c14e1f0a" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/RevenueScheduleofRevenueRecognizedonaNetBasisandonaGrossBasisDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#RevenueScheduleofRevenueRecognizedonaNetBasisandonaGrossBasisDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/RevenueScheduleofRevenueRecognizedonaNetBasisandonaGrossBasisDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_4e09cf0b-755d-4ced-a50b-05989d6bd026" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_3659826c-9c70-4108-accc-eaaabf17e7ea" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_4e09cf0b-755d-4ced-a50b-05989d6bd026" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_3659826c-9c70-4108-accc-eaaabf17e7ea" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_BasisOfRevenueRecognitionAxis_5a5cfe9e-5f17-4d48-919a-9e660bdbf482" xlink:href="mgni-20251231.xsd#mgni_BasisOfRevenueRecognitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_3659826c-9c70-4108-accc-eaaabf17e7ea" xlink:to="loc_mgni_BasisOfRevenueRecognitionAxis_5a5cfe9e-5f17-4d48-919a-9e660bdbf482" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_BasisOfRevenueRecognitionDomain_5a5cfe9e-5f17-4d48-919a-9e660bdbf482_default" xlink:href="mgni-20251231.xsd#mgni_BasisOfRevenueRecognitionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_mgni_BasisOfRevenueRecognitionAxis_5a5cfe9e-5f17-4d48-919a-9e660bdbf482" xlink:to="loc_mgni_BasisOfRevenueRecognitionDomain_5a5cfe9e-5f17-4d48-919a-9e660bdbf482_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_BasisOfRevenueRecognitionDomain_9e78f70f-6425-4074-8c9f-19c8e84edbaa" xlink:href="mgni-20251231.xsd#mgni_BasisOfRevenueRecognitionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_mgni_BasisOfRevenueRecognitionAxis_5a5cfe9e-5f17-4d48-919a-9e660bdbf482" xlink:to="loc_mgni_BasisOfRevenueRecognitionDomain_9e78f70f-6425-4074-8c9f-19c8e84edbaa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_NetRevenueMember_e091400a-a931-431d-aef3-451ec95802fd" xlink:href="mgni-20251231.xsd#mgni_NetRevenueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_BasisOfRevenueRecognitionDomain_9e78f70f-6425-4074-8c9f-19c8e84edbaa" xlink:to="loc_mgni_NetRevenueMember_e091400a-a931-431d-aef3-451ec95802fd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_GrossRevenueMember_588566e3-bb23-4027-aa07-218aaa5e3b99" xlink:href="mgni-20251231.xsd#mgni_GrossRevenueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_BasisOfRevenueRecognitionDomain_9e78f70f-6425-4074-8c9f-19c8e84edbaa" xlink:to="loc_mgni_GrossRevenueMember_588566e3-bb23-4027-aa07-218aaa5e3b99" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_10335a3a-9a41-4b65-a0fb-0cd4f474c185" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_3659826c-9c70-4108-accc-eaaabf17e7ea" xlink:to="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_10335a3a-9a41-4b65-a0fb-0cd4f474c185" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_10335a3a-9a41-4b65-a0fb-0cd4f474c185_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_10335a3a-9a41-4b65-a0fb-0cd4f474c185" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_10335a3a-9a41-4b65-a0fb-0cd4f474c185_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_b5086b04-8a31-4a5e-a22f-040954ce5678" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_10335a3a-9a41-4b65-a0fb-0cd4f474c185" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_b5086b04-8a31-4a5e-a22f-040954ce5678" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerMember_13f92997-c502-42ed-88e2-dda5a28ba032" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_b5086b04-8a31-4a5e-a22f-040954ce5678" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerMember_13f92997-c502-42ed-88e2-dda5a28ba032" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByTypeAxis_84fe77da-74dc-44f7-8db4-cdd36f1930ec" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_3659826c-9c70-4108-accc-eaaabf17e7ea" xlink:to="loc_us-gaap_ConcentrationRiskByTypeAxis_84fe77da-74dc-44f7-8db4-cdd36f1930ec" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_84fe77da-74dc-44f7-8db4-cdd36f1930ec_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_84fe77da-74dc-44f7-8db4-cdd36f1930ec" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_84fe77da-74dc-44f7-8db4-cdd36f1930ec_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_6c9ac786-78ba-42c9-af13-8a6b52289c3f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_84fe77da-74dc-44f7-8db4-cdd36f1930ec" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_6c9ac786-78ba-42c9-af13-8a6b52289c3f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ConcentrationOfBasisOfRevenueRecognitionMember_e8bf7fa8-0475-4211-b14b-c3b4b8683f62" xlink:href="mgni-20251231.xsd#mgni_ConcentrationOfBasisOfRevenueRecognitionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_6c9ac786-78ba-42c9-af13-8a6b52289c3f" xlink:to="loc_mgni_ConcentrationOfBasisOfRevenueRecognitionMember_e8bf7fa8-0475-4211-b14b-c3b4b8683f62" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_3e74a179-e45a-4d59-a6bb-c9a02285bd9d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_4e09cf0b-755d-4ced-a50b-05989d6bd026" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_3e74a179-e45a-4d59-a6bb-c9a02285bd9d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskPercentage1_5f56a747-42b6-4815-96b9-0dbd4466d39c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskPercentage1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_4e09cf0b-755d-4ced-a50b-05989d6bd026" xlink:to="loc_us-gaap_ConcentrationRiskPercentage1_5f56a747-42b6-4815-96b9-0dbd4466d39c" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/RevenueScheduleofRevenueDisaggregatedbySalesDistributionChannelDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#RevenueScheduleofRevenueDisaggregatedbySalesDistributionChannelDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/RevenueScheduleofRevenueDisaggregatedbySalesDistributionChannelDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_5ae289a0-e79b-485d-8807-64e57f940986" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_3713920b-3824-4fd3-a581-58d6c78b06f9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_5ae289a0-e79b-485d-8807-64e57f940986" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_3713920b-3824-4fd3-a581-58d6c78b06f9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerSalesChannelAxis_3d570a08-ba1c-4159-aaf3-0b650626fcd9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ContractWithCustomerSalesChannelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_3713920b-3824-4fd3-a581-58d6c78b06f9" xlink:to="loc_us-gaap_ContractWithCustomerSalesChannelAxis_3d570a08-ba1c-4159-aaf3-0b650626fcd9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerSalesChannelDomain_3d570a08-ba1c-4159-aaf3-0b650626fcd9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ContractWithCustomerSalesChannelDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ContractWithCustomerSalesChannelAxis_3d570a08-ba1c-4159-aaf3-0b650626fcd9" xlink:to="loc_us-gaap_ContractWithCustomerSalesChannelDomain_3d570a08-ba1c-4159-aaf3-0b650626fcd9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerSalesChannelDomain_c6426416-5c35-4745-b329-5c4bf920d4f9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ContractWithCustomerSalesChannelDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ContractWithCustomerSalesChannelAxis_3d570a08-ba1c-4159-aaf3-0b650626fcd9" xlink:to="loc_us-gaap_ContractWithCustomerSalesChannelDomain_c6426416-5c35-4745-b329-5c4bf920d4f9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ConnectedTelevisionMember_4dd908a3-1c6d-45b4-af70-8a16b947e87d" xlink:href="mgni-20251231.xsd#mgni_ConnectedTelevisionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ContractWithCustomerSalesChannelDomain_c6426416-5c35-4745-b329-5c4bf920d4f9" xlink:to="loc_mgni_ConnectedTelevisionMember_4dd908a3-1c6d-45b4-af70-8a16b947e87d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_MobileMember_3919932e-c147-469e-bfc9-81fbde13b7d6" xlink:href="mgni-20251231.xsd#mgni_MobileMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ContractWithCustomerSalesChannelDomain_c6426416-5c35-4745-b329-5c4bf920d4f9" xlink:to="loc_mgni_MobileMember_3919932e-c147-469e-bfc9-81fbde13b7d6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_DesktopMember_c78d2220-39ab-4cdc-ac1c-616c7e24feac" xlink:href="mgni-20251231.xsd#mgni_DesktopMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ContractWithCustomerSalesChannelDomain_c6426416-5c35-4745-b329-5c4bf920d4f9" xlink:to="loc_mgni_DesktopMember_c78d2220-39ab-4cdc-ac1c-616c7e24feac" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_ef341fdd-b8a8-4623-9d34-7009284b6478" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_3713920b-3824-4fd3-a581-58d6c78b06f9" xlink:to="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_ef341fdd-b8a8-4623-9d34-7009284b6478" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_ef341fdd-b8a8-4623-9d34-7009284b6478_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_ef341fdd-b8a8-4623-9d34-7009284b6478" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_ef341fdd-b8a8-4623-9d34-7009284b6478_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_57625ffa-4787-4b51-be7a-4e1ee0464074" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_ef341fdd-b8a8-4623-9d34-7009284b6478" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_57625ffa-4787-4b51-be7a-4e1ee0464074" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerMember_68064d36-0f55-4c2e-8a0c-b24a1cc758d4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_57625ffa-4787-4b51-be7a-4e1ee0464074" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerMember_68064d36-0f55-4c2e-8a0c-b24a1cc758d4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByTypeAxis_03fc5547-21d9-46c1-b357-5197adf473f5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_3713920b-3824-4fd3-a581-58d6c78b06f9" xlink:to="loc_us-gaap_ConcentrationRiskByTypeAxis_03fc5547-21d9-46c1-b357-5197adf473f5" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_03fc5547-21d9-46c1-b357-5197adf473f5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_03fc5547-21d9-46c1-b357-5197adf473f5" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_03fc5547-21d9-46c1-b357-5197adf473f5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_6a2a4bbf-baf9-47e4-9e28-270d8c153abc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_03fc5547-21d9-46c1-b357-5197adf473f5" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_6a2a4bbf-baf9-47e4-9e28-270d8c153abc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProductConcentrationRiskMember_4cc39710-147c-482f-9a56-e3abcc588795" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProductConcentrationRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_6a2a4bbf-baf9-47e4-9e28-270d8c153abc" xlink:to="loc_us-gaap_ProductConcentrationRiskMember_4cc39710-147c-482f-9a56-e3abcc588795" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_d935f23a-ff44-449d-a7de-8f1bbbd2fde0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_5ae289a0-e79b-485d-8807-64e57f940986" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_d935f23a-ff44-449d-a7de-8f1bbbd2fde0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskPercentage1_64cffec1-641f-430c-a695-5265b5ec85dd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskPercentage1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_5ae289a0-e79b-485d-8807-64e57f940986" xlink:to="loc_us-gaap_ConcentrationRiskPercentage1_64cffec1-641f-430c-a695-5265b5ec85dd" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/RevenueScheduleofRevenueDisaggregatedbyGeographicLocationDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#RevenueScheduleofRevenueDisaggregatedbyGeographicLocationDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/RevenueScheduleofRevenueDisaggregatedbyGeographicLocationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_808804e4-1d77-429d-9cdb-315187bc1c37" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_0eb15781-27eb-4b13-840b-0f8a21f2050a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_808804e4-1d77-429d-9cdb-315187bc1c37" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_0eb15781-27eb-4b13-840b-0f8a21f2050a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_7bb234b0-d6a5-48da-85ef-d42f7a16cad5" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_0eb15781-27eb-4b13-840b-0f8a21f2050a" xlink:to="loc_srt_StatementGeographicalAxis_7bb234b0-d6a5-48da-85ef-d42f7a16cad5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_7bb234b0-d6a5-48da-85ef-d42f7a16cad5_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_7bb234b0-d6a5-48da-85ef-d42f7a16cad5" xlink:to="loc_srt_SegmentGeographicalDomain_7bb234b0-d6a5-48da-85ef-d42f7a16cad5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_ef388627-9cf0-4854-824d-4343348a556b" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_7bb234b0-d6a5-48da-85ef-d42f7a16cad5" xlink:to="loc_srt_SegmentGeographicalDomain_ef388627-9cf0-4854-824d-4343348a556b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_69301bf8-4fda-4937-ab85-de960daa1ca9" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_ef388627-9cf0-4854-824d-4343348a556b" xlink:to="loc_country_US_69301bf8-4fda-4937-ab85-de960daa1ca9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonUsMember_dda6c171-8190-4920-9128-e1f135c2d3c4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NonUsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_ef388627-9cf0-4854-824d-4343348a556b" xlink:to="loc_us-gaap_NonUsMember_dda6c171-8190-4920-9128-e1f135c2d3c4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_c7b1b3d1-861c-4a55-b25b-0b2d489e1f72" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_0eb15781-27eb-4b13-840b-0f8a21f2050a" xlink:to="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_c7b1b3d1-861c-4a55-b25b-0b2d489e1f72" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_c7b1b3d1-861c-4a55-b25b-0b2d489e1f72_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_c7b1b3d1-861c-4a55-b25b-0b2d489e1f72" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_c7b1b3d1-861c-4a55-b25b-0b2d489e1f72_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_938839e4-6fee-4769-922c-94c9ca13cded" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_c7b1b3d1-861c-4a55-b25b-0b2d489e1f72" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_938839e4-6fee-4769-922c-94c9ca13cded" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerMember_4595c565-0659-4ec8-a3c0-304e2c4e9ba5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_938839e4-6fee-4769-922c-94c9ca13cded" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerMember_4595c565-0659-4ec8-a3c0-304e2c4e9ba5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByTypeAxis_c040cb31-f162-4289-8466-0ad7e93a4274" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_0eb15781-27eb-4b13-840b-0f8a21f2050a" xlink:to="loc_us-gaap_ConcentrationRiskByTypeAxis_c040cb31-f162-4289-8466-0ad7e93a4274" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_c040cb31-f162-4289-8466-0ad7e93a4274_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_c040cb31-f162-4289-8466-0ad7e93a4274" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_c040cb31-f162-4289-8466-0ad7e93a4274_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_b8664e1a-af28-41c8-be58-a28e6ba93fd2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_c040cb31-f162-4289-8466-0ad7e93a4274" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_b8664e1a-af28-41c8-be58-a28e6ba93fd2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GeographicConcentrationRiskMember_c9c16569-bf10-492d-894e-920665001ea4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GeographicConcentrationRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_b8664e1a-af28-41c8-be58-a28e6ba93fd2" xlink:to="loc_us-gaap_GeographicConcentrationRiskMember_c9c16569-bf10-492d-894e-920665001ea4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_52877c16-c33f-4d9d-bc27-195840cc9def" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_808804e4-1d77-429d-9cdb-315187bc1c37" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_52877c16-c33f-4d9d-bc27-195840cc9def" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskPercentage1_bce0ec21-6d02-424d-8478-271648cd5c46" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskPercentage1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_808804e4-1d77-429d-9cdb-315187bc1c37" xlink:to="loc_us-gaap_ConcentrationRiskPercentage1_bce0ec21-6d02-424d-8478-271648cd5c46" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/RevenueNarrativeDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#RevenueNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/RevenueNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_74e1df91-4ead-408c-a4eb-29927c33bd9b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_2a757b66-de04-4575-9ad2-7ead33fd05d5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_74e1df91-4ead-408c-a4eb-29927c33bd9b" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_2a757b66-de04-4575-9ad2-7ead33fd05d5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MajorCustomersAxis_45604c9d-92bc-41e0-a1b0-ba5ce19cb6c8" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MajorCustomersAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_2a757b66-de04-4575-9ad2-7ead33fd05d5" xlink:to="loc_srt_MajorCustomersAxis_45604c9d-92bc-41e0-a1b0-ba5ce19cb6c8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NameOfMajorCustomerDomain_45604c9d-92bc-41e0-a1b0-ba5ce19cb6c8_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_NameOfMajorCustomerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_MajorCustomersAxis_45604c9d-92bc-41e0-a1b0-ba5ce19cb6c8" xlink:to="loc_srt_NameOfMajorCustomerDomain_45604c9d-92bc-41e0-a1b0-ba5ce19cb6c8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NameOfMajorCustomerDomain_83e6c7d8-2618-4ca9-8c9d-fa8b82064187" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_NameOfMajorCustomerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_MajorCustomersAxis_45604c9d-92bc-41e0-a1b0-ba5ce19cb6c8" xlink:to="loc_srt_NameOfMajorCustomerDomain_83e6c7d8-2618-4ca9-8c9d-fa8b82064187" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_OneBuyerMember_e40c9901-c3ec-4f79-85fd-04b33ff7fad1" xlink:href="mgni-20251231.xsd#mgni_OneBuyerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_83e6c7d8-2618-4ca9-8c9d-fa8b82064187" xlink:to="loc_mgni_OneBuyerMember_e40c9901-c3ec-4f79-85fd-04b33ff7fad1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_OneCustomerMember_b3b65960-3750-4fee-9b0d-083b3b22c25e" xlink:href="mgni-20251231.xsd#mgni_OneCustomerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_83e6c7d8-2618-4ca9-8c9d-fa8b82064187" xlink:to="loc_mgni_OneCustomerMember_b3b65960-3750-4fee-9b0d-083b3b22c25e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_RevenuePaymentTerms_a2b73d5d-9634-4db2-804a-47e8723d533c" xlink:href="mgni-20251231.xsd#mgni_RevenuePaymentTerms"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_74e1df91-4ead-408c-a4eb-29927c33bd9b" xlink:to="loc_mgni_RevenuePaymentTerms_a2b73d5d-9634-4db2-804a-47e8723d533c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllowanceForDoubtfulAccountsReceivable_13151a03-0db1-474a-9eaf-fb2a668a7f06" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AllowanceForDoubtfulAccountsReceivable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_74e1df91-4ead-408c-a4eb-29927c33bd9b" xlink:to="loc_us-gaap_AllowanceForDoubtfulAccountsReceivable_13151a03-0db1-474a-9eaf-fb2a668a7f06" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_AccountsPayableSeller_7d6876d6-4a06-462c-8bee-ed0c5dcbad39" xlink:href="mgni-20251231.xsd#mgni_AccountsPayableSeller"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_74e1df91-4ead-408c-a4eb-29927c33bd9b" xlink:to="loc_mgni_AccountsPayableSeller_7d6876d6-4a06-462c-8bee-ed0c5dcbad39" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableWriteOffs_83c69eb3-dd45-438e-8fb1-72c89d42207d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AllowanceForDoubtfulAccountsReceivableWriteOffs"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_74e1df91-4ead-408c-a4eb-29927c33bd9b" xlink:to="loc_us-gaap_AllowanceForDoubtfulAccountsReceivableWriteOffs_83c69eb3-dd45-438e-8fb1-72c89d42207d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_AccountsReceivableCreditLossExpenseReversalGross_482f8586-cc6a-44c9-af5b-0e7d9dcc631b" xlink:href="mgni-20251231.xsd#mgni_AccountsReceivableCreditLossExpenseReversalGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_74e1df91-4ead-408c-a4eb-29927c33bd9b" xlink:to="loc_mgni_AccountsReceivableCreditLossExpenseReversalGross_482f8586-cc6a-44c9-af5b-0e7d9dcc631b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_IncreaseDecreaseInAccountsPayableSeller_783805aa-2a91-4982-ac8c-e34d7170ad08" xlink:href="mgni-20251231.xsd#mgni_IncreaseDecreaseInAccountsPayableSeller"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_74e1df91-4ead-408c-a4eb-29927c33bd9b" xlink:to="loc_mgni_IncreaseDecreaseInAccountsPayableSeller_783805aa-2a91-4982-ac8c-e34d7170ad08" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProvisionForDoubtfulAccounts_87ac53fc-d19e-4ade-959e-8ed8b31f3483" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProvisionForDoubtfulAccounts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_74e1df91-4ead-408c-a4eb-29927c33bd9b" xlink:to="loc_us-gaap_ProvisionForDoubtfulAccounts_87ac53fc-d19e-4ade-959e-8ed8b31f3483" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/FairValueMeasurementsScheduleofFinancialInstrumentsDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#FairValueMeasurementsScheduleofFinancialInstrumentsDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/FairValueMeasurementsScheduleofFinancialInstrumentsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_a2b8388a-5659-4715-b70b-cadc2798122e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_a118c01f-a85e-41b4-a691-83967ec9a805" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_a2b8388a-5659-4715-b70b-cadc2798122e" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_a118c01f-a85e-41b4-a691-83967ec9a805" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_1634f82a-92e6-406c-a1df-1379d1b73dca" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByMeasurementFrequencyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_a118c01f-a85e-41b4-a691-83967ec9a805" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_1634f82a-92e6-406c-a1df-1379d1b73dca" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_1634f82a-92e6-406c-a1df-1379d1b73dca_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_1634f82a-92e6-406c-a1df-1379d1b73dca" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_1634f82a-92e6-406c-a1df-1379d1b73dca_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_bf76193d-2730-4ea5-9099-8504751dcdd1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_1634f82a-92e6-406c-a1df-1379d1b73dca" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_bf76193d-2730-4ea5-9099-8504751dcdd1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsRecurringMember_6c708572-d205-486f-a737-da05002fd332" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsRecurringMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_bf76193d-2730-4ea5-9099-8504751dcdd1" xlink:to="loc_us-gaap_FairValueMeasurementsRecurringMember_6c708572-d205-486f-a737-da05002fd332" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_6085b733-3c67-48ef-8447-28f99a1a59af" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_a118c01f-a85e-41b4-a691-83967ec9a805" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_6085b733-3c67-48ef-8447-28f99a1a59af" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_6085b733-3c67-48ef-8447-28f99a1a59af_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_6085b733-3c67-48ef-8447-28f99a1a59af" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_6085b733-3c67-48ef-8447-28f99a1a59af_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_45f193e6-08d9-4880-a448-0c68df6f6cff" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_6085b733-3c67-48ef-8447-28f99a1a59af" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_45f193e6-08d9-4880-a448-0c68df6f6cff" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_918ea831-d271-4ca5-90c9-344714c24d35" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_45f193e6-08d9-4880-a448-0c68df6f6cff" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_918ea831-d271-4ca5-90c9-344714c24d35" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_2c56acfe-c68c-40c6-a445-eceaa1f21738" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_45f193e6-08d9-4880-a448-0c68df6f6cff" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_2c56acfe-c68c-40c6-a445-eceaa1f21738" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_4fe056a1-d60d-4df0-81d4-3bec5ea5321e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_45f193e6-08d9-4880-a448-0c68df6f6cff" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_4fe056a1-d60d-4df0-81d4-3bec5ea5321e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_50e3208f-3a9a-468e-ab09-84c0815373db" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndCashEquivalentsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_a2b8388a-5659-4715-b70b-cadc2798122e" xlink:to="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_50e3208f-3a9a-468e-ab09-84c0815373db" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/FairValueMeasurementsNarrativeDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#FairValueMeasurementsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/FairValueMeasurementsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_bb82981a-32e0-4d69-a478-d6720cde4da5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_0c3eaa9d-23a6-4f0c-905a-70da21465375" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_bb82981a-32e0-4d69-a478-d6720cde4da5" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_0c3eaa9d-23a6-4f0c-905a-70da21465375" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_91d623f8-2285-48cb-abbe-477257c9aba7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByMeasurementFrequencyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_0c3eaa9d-23a6-4f0c-905a-70da21465375" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_91d623f8-2285-48cb-abbe-477257c9aba7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_91d623f8-2285-48cb-abbe-477257c9aba7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_91d623f8-2285-48cb-abbe-477257c9aba7" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_91d623f8-2285-48cb-abbe-477257c9aba7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_0dc9cb12-6f3c-4086-9847-1e8fbb3c86cd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_91d623f8-2285-48cb-abbe-477257c9aba7" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_0dc9cb12-6f3c-4086-9847-1e8fbb3c86cd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsRecurringMember_9be60e58-aa1f-4b1f-8dec-1ee5311bf7e2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsRecurringMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_0dc9cb12-6f3c-4086-9847-1e8fbb3c86cd" xlink:to="loc_us-gaap_FairValueMeasurementsRecurringMember_9be60e58-aa1f-4b1f-8dec-1ee5311bf7e2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_ca63d5f5-ef0d-49a9-b0e5-2701860c4b11" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_0c3eaa9d-23a6-4f0c-905a-70da21465375" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_ca63d5f5-ef0d-49a9-b0e5-2701860c4b11" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_ca63d5f5-ef0d-49a9-b0e5-2701860c4b11_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_ca63d5f5-ef0d-49a9-b0e5-2701860c4b11" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_ca63d5f5-ef0d-49a9-b0e5-2701860c4b11_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_f462bb76-4c32-457b-a0b0-e557330821ce" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_ca63d5f5-ef0d-49a9-b0e5-2701860c4b11" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_f462bb76-4c32-457b-a0b0-e557330821ce" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_724a0d97-ee5a-4652-ad6c-b0bcc9e65f84" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_f462bb76-4c32-457b-a0b0-e557330821ce" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_724a0d97-ee5a-4652-ad6c-b0bcc9e65f84" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_b4c478b1-1644-46fb-a4c0-de357717a8d4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_0c3eaa9d-23a6-4f0c-905a-70da21465375" xlink:to="loc_us-gaap_DebtInstrumentAxis_b4c478b1-1644-46fb-a4c0-de357717a8d4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_b4c478b1-1644-46fb-a4c0-de357717a8d4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_b4c478b1-1644-46fb-a4c0-de357717a8d4" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_b4c478b1-1644-46fb-a4c0-de357717a8d4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_8e7adfaa-8f1b-4fd2-8368-d6d85b84703a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_b4c478b1-1644-46fb-a4c0-de357717a8d4" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_8e7adfaa-8f1b-4fd2-8368-d6d85b84703a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_CreditAgreements2021And2024Member_af41cee9-000b-44d5-a4c9-9ae509457be6" xlink:href="mgni-20251231.xsd#mgni_CreditAgreements2021And2024Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_8e7adfaa-8f1b-4fd2-8368-d6d85b84703a" xlink:to="loc_mgni_CreditAgreements2021And2024Member_af41cee9-000b-44d5-a4c9-9ae509457be6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_84126773-1666-4c33-aac3-6a4cce773d42" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_0c3eaa9d-23a6-4f0c-905a-70da21465375" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_84126773-1666-4c33-aac3-6a4cce773d42" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_84126773-1666-4c33-aac3-6a4cce773d42_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_84126773-1666-4c33-aac3-6a4cce773d42" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_84126773-1666-4c33-aac3-6a4cce773d42_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_b9d00d2d-1dd9-43bc-ae75-f05de9e09c6c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_84126773-1666-4c33-aac3-6a4cce773d42" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_b9d00d2d-1dd9-43bc-ae75-f05de9e09c6c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_05488669-5141-4b00-8674-eed29340b076" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_b9d00d2d-1dd9-43bc-ae75-f05de9e09c6c" xlink:to="loc_us-gaap_LineOfCreditMember_05488669-5141-4b00-8674-eed29340b076" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_178cfe02-d686-4e6a-841f-6f4789c5471e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_0c3eaa9d-23a6-4f0c-905a-70da21465375" xlink:to="loc_us-gaap_CreditFacilityAxis_178cfe02-d686-4e6a-841f-6f4789c5471e" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_178cfe02-d686-4e6a-841f-6f4789c5471e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_178cfe02-d686-4e6a-841f-6f4789c5471e" xlink:to="loc_us-gaap_CreditFacilityDomain_178cfe02-d686-4e6a-841f-6f4789c5471e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_0c7ba68d-f666-4627-ad25-4ea6cc2256f3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_178cfe02-d686-4e6a-841f-6f4789c5471e" xlink:to="loc_us-gaap_CreditFacilityDomain_0c7ba68d-f666-4627-ad25-4ea6cc2256f3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_cef76445-45d3-4360-a89a-ee9e537bd2fe" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_0c7ba68d-f666-4627-ad25-4ea6cc2256f3" xlink:to="loc_us-gaap_SecuredDebtMember_cef76445-45d3-4360-a89a-ee9e537bd2fe" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_21c52580-e778-4130-925e-4c0ce06c4dfa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndCashEquivalentsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_bb82981a-32e0-4d69-a478-d6720cde4da5" xlink:to="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_21c52580-e778-4130-925e-4c0ce06c4dfa" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtFairValueDisclosures_d064f21e-c6db-451b-829b-4ade2a35bc57" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConvertibleDebtFairValueDisclosures"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_bb82981a-32e0-4d69-a478-d6720cde4da5" xlink:to="loc_us-gaap_ConvertibleDebtFairValueDisclosures_d064f21e-c6db-451b-829b-4ade2a35bc57" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtFairValue_20c32a8e-645b-41b2-8b18-602909eeff07" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_bb82981a-32e0-4d69-a478-d6720cde4da5" xlink:to="loc_us-gaap_LongTermDebtFairValue_20c32a8e-645b-41b2-8b18-602909eeff07" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/PropertyandEquipmentScheduleofPropertyandEquipmentbyMajorClassDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#PropertyandEquipmentScheduleofPropertyandEquipmentbyMajorClassDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/PropertyandEquipmentScheduleofPropertyandEquipmentbyMajorClassDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_1aa79e8d-0386-497b-a0ff-f19196747227" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_2d5883ec-60ad-4e39-9fbb-23a3c9d14aac" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_1aa79e8d-0386-497b-a0ff-f19196747227" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_2d5883ec-60ad-4e39-9fbb-23a3c9d14aac" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_a5cc6117-d203-421c-a6fd-655551877221" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_2d5883ec-60ad-4e39-9fbb-23a3c9d14aac" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_a5cc6117-d203-421c-a6fd-655551877221" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_a5cc6117-d203-421c-a6fd-655551877221_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_a5cc6117-d203-421c-a6fd-655551877221" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_a5cc6117-d203-421c-a6fd-655551877221_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_8d22ac93-5b41-4fa4-a99d-8ca09dc2bd5c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_a5cc6117-d203-421c-a6fd-655551877221" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_8d22ac93-5b41-4fa4-a99d-8ca09dc2bd5c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ComputerEquipmentMember_36ce51f8-7d94-4b71-8ea3-c10a1581253f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ComputerEquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_8d22ac93-5b41-4fa4-a99d-8ca09dc2bd5c" xlink:to="loc_us-gaap_ComputerEquipmentMember_36ce51f8-7d94-4b71-8ea3-c10a1581253f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FurnitureAndFixturesMember_4cbf41de-dc2a-4c1f-b211-1b212da08a61" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FurnitureAndFixturesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_8d22ac93-5b41-4fa4-a99d-8ca09dc2bd5c" xlink:to="loc_us-gaap_FurnitureAndFixturesMember_4cbf41de-dc2a-4c1f-b211-1b212da08a61" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseholdImprovementsMember_4911f687-d1d4-43ce-a70c-9e1c6d8e307e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LeaseholdImprovementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_8d22ac93-5b41-4fa4-a99d-8ca09dc2bd5c" xlink:to="loc_us-gaap_LeaseholdImprovementsMember_4911f687-d1d4-43ce-a70c-9e1c6d8e307e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SoftwareAndSoftwareDevelopmentCostsMember_fd967279-48b2-4688-9bd5-9c68de03f3dd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SoftwareAndSoftwareDevelopmentCostsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_8d22ac93-5b41-4fa4-a99d-8ca09dc2bd5c" xlink:to="loc_us-gaap_SoftwareAndSoftwareDevelopmentCostsMember_fd967279-48b2-4688-9bd5-9c68de03f3dd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentGross_019b9989-f6ad-7a04-bfb4-21d505279da5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_1aa79e8d-0386-497b-a0ff-f19196747227" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentGross_019b9989-f6ad-7a04-bfb4-21d505279da5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_26e0b770-36cc-4386-a625-5db40c54bec5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_1aa79e8d-0386-497b-a0ff-f19196747227" xlink:to="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_26e0b770-36cc-4386-a625-5db40c54bec5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_844b9f32-a32f-4a84-a876-f49e3852b937" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_1aa79e8d-0386-497b-a0ff-f19196747227" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_844b9f32-a32f-4a84-a876-f49e3852b937" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/PropertyandEquipmentScheduleofPropertyandEquipmentbyGeographicalRegionDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#PropertyandEquipmentScheduleofPropertyandEquipmentbyGeographicalRegionDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/PropertyandEquipmentScheduleofPropertyandEquipmentbyGeographicalRegionDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_0b057251-7981-476d-8225-c407e755ca2e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_5dc95a93-0617-474e-b7cc-ffe2afa26114" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_0b057251-7981-476d-8225-c407e755ca2e" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_5dc95a93-0617-474e-b7cc-ffe2afa26114" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_08ee4d52-db4d-4845-9c4d-af3adc2bce26" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_5dc95a93-0617-474e-b7cc-ffe2afa26114" xlink:to="loc_srt_StatementGeographicalAxis_08ee4d52-db4d-4845-9c4d-af3adc2bce26" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_08ee4d52-db4d-4845-9c4d-af3adc2bce26_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_08ee4d52-db4d-4845-9c4d-af3adc2bce26" xlink:to="loc_srt_SegmentGeographicalDomain_08ee4d52-db4d-4845-9c4d-af3adc2bce26_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_bc00222e-cb8f-4aca-917f-d7d6a5c527eb" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_08ee4d52-db4d-4845-9c4d-af3adc2bce26" xlink:to="loc_srt_SegmentGeographicalDomain_bc00222e-cb8f-4aca-917f-d7d6a5c527eb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_d69bea1e-8733-4baf-bab7-c5141dd3bb17" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_bc00222e-cb8f-4aca-917f-d7d6a5c527eb" xlink:to="loc_country_US_d69bea1e-8733-4baf-bab7-c5141dd3bb17" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonUsMember_a1e7b9ba-d992-411d-925e-fc40ddf5e51d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NonUsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_bc00222e-cb8f-4aca-917f-d7d6a5c527eb" xlink:to="loc_us-gaap_NonUsMember_a1e7b9ba-d992-411d-925e-fc40ddf5e51d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_e9e6fd68-8122-46f4-b18b-e5ef8be60b92" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_0b057251-7981-476d-8225-c407e755ca2e" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_e9e6fd68-8122-46f4-b18b-e5ef8be60b92" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/BusinessCombinationsDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#BusinessCombinationsDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/BusinessCombinationsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_019aff21-69db-7ea7-a4a5-dd1de192ea66" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_b7d5ded3-820b-44dd-b07a-8552b0d3b356" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019aff21-69db-7ea7-a4a5-dd1de192ea66" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_b7d5ded3-820b-44dd-b07a-8552b0d3b356" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_019aff21-69db-79ed-89d8-91a473a56279" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_b7d5ded3-820b-44dd-b07a-8552b0d3b356" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_019aff21-69db-79ed-89d8-91a473a56279" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019aff21-69db-79ed-89d8-91a473a56279_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_019aff21-69db-79ed-89d8-91a473a56279" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019aff21-69db-79ed-89d8-91a473a56279_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019aff21-69db-7c70-897a-7c906dc9def8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_019aff21-69db-79ed-89d8-91a473a56279" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019aff21-69db-7c70-897a-7c906dc9def8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_StreamraiInc.Member_019aff21-69db-7f7c-b833-ccefa9490c9b" xlink:href="mgni-20251231.xsd#mgni_StreamraiInc.Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019aff21-69db-7c70-897a-7c906dc9def8" xlink:to="loc_mgni_StreamraiInc.Member_019aff21-69db-7f7c-b833-ccefa9490c9b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019aff21-69db-795f-a312-ed6c2b81ac43" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_b7d5ded3-820b-44dd-b07a-8552b0d3b356" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019aff21-69db-795f-a312-ed6c2b81ac43" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019aff21-69db-795f-a312-ed6c2b81ac43_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019aff21-69db-795f-a312-ed6c2b81ac43" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019aff21-69db-795f-a312-ed6c2b81ac43_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019aff21-69db-7513-a11e-9a273b29ae65" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019aff21-69db-795f-a312-ed6c2b81ac43" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019aff21-69db-7513-a11e-9a273b29ae65" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TechnologyBasedIntangibleAssetsMember_019aff21-69db-7a83-98ff-cea36c2f5fba" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TechnologyBasedIntangibleAssetsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019aff21-69db-7513-a11e-9a273b29ae65" xlink:to="loc_us-gaap_TechnologyBasedIntangibleAssetsMember_019aff21-69db-7a83-98ff-cea36c2f5fba" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncompeteAgreementsMember_019aff21-69db-731e-8308-ae668378cb4b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NoncompeteAgreementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019aff21-69db-7513-a11e-9a273b29ae65" xlink:to="loc_us-gaap_NoncompeteAgreementsMember_019aff21-69db-731e-8308-ae668378cb4b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_019aff21-69db-70c1-a9f1-c5b5cb1051f1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019aff21-69db-7513-a11e-9a273b29ae65" xlink:to="loc_us-gaap_CustomerRelationshipsMember_019aff21-69db-70c1-a9f1-c5b5cb1051f1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TrademarksAndTradeNamesMember_019aff21-69db-7bda-93da-21da34b969f8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TrademarksAndTradeNamesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019aff21-69db-7513-a11e-9a273b29ae65" xlink:to="loc_us-gaap_TrademarksAndTradeNamesMember_019aff21-69db-7bda-93da-21da34b969f8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019aff21-69db-7887-a412-d5411c686ad1" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_b7d5ded3-820b-44dd-b07a-8552b0d3b356" xlink:to="loc_srt_RangeAxis_019aff21-69db-7887-a412-d5411c686ad1" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019aff21-69db-7887-a412-d5411c686ad1_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019aff21-69db-7887-a412-d5411c686ad1" xlink:to="loc_srt_RangeMember_019aff21-69db-7887-a412-d5411c686ad1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019aff21-69db-7df4-90b8-38425dd7dcd4" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019aff21-69db-7887-a412-d5411c686ad1" xlink:to="loc_srt_RangeMember_019aff21-69db-7df4-90b8-38425dd7dcd4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019aff21-69db-7c69-affe-e4e69d8919fe" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019aff21-69db-7df4-90b8-38425dd7dcd4" xlink:to="loc_srt_MinimumMember_019aff21-69db-7c69-affe-e4e69d8919fe" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019aff21-69db-7da5-89ab-3df3d893c2f3" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019aff21-69db-7df4-90b8-38425dd7dcd4" xlink:to="loc_srt_MaximumMember_019aff21-69db-7da5-89ab-3df3d893c2f3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferred1_019aff21-69db-7149-8df9-795ceda7b511" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationConsiderationTransferred1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019aff21-69db-7ea7-a4a5-dd1de192ea66" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferred1_019aff21-69db-7149-8df9-795ceda7b511" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesGross_019aff21-69db-75db-8b09-efce8b367b7f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquireBusinessesGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019aff21-69db-7ea7-a4a5-dd1de192ea66" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesGross_019aff21-69db-75db-8b09-efce8b367b7f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationLiability_019aff21-69db-7c80-b4ea-d59642db3c30" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationContingentConsiderationLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019aff21-69db-7ea7-a4a5-dd1de192ea66" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationLiability_019aff21-69db-7c80-b4ea-d59642db3c30" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_BusinessCombinationContingentConsiderationLiabilityEarnoutPeriod_019aff21-69db-7d2a-8671-18f906de171e" xlink:href="mgni-20251231.xsd#mgni_BusinessCombinationContingentConsiderationLiabilityEarnoutPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019aff21-69db-7ea7-a4a5-dd1de192ea66" xlink:to="loc_mgni_BusinessCombinationContingentConsiderationLiabilityEarnoutPeriod_019aff21-69db-7d2a-8671-18f906de171e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinitelivedIntangibleAssetsAcquired1_019aff21-69db-7e31-860b-7e99ed245b3f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinitelivedIntangibleAssetsAcquired1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019aff21-69db-7ea7-a4a5-dd1de192ea66" xlink:to="loc_us-gaap_FinitelivedIntangibleAssetsAcquired1_019aff21-69db-7e31-860b-7e99ed245b3f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife_019aff21-69db-743e-9d14-2dd764a3e5d3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019aff21-69db-7ea7-a4a5-dd1de192ea66" xlink:to="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife_019aff21-69db-743e-9d14-2dd764a3e5d3" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_019aff21-69db-7da2-bce3-00de23258938" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019aff21-69db-7ea7-a4a5-dd1de192ea66" xlink:to="loc_us-gaap_Goodwill_019aff21-69db-7da2-bce3-00de23258938" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities_019aff21-69db-7d82-bedf-868a512331d6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019aff21-69db-7ea7-a4a5-dd1de192ea66" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilities_019aff21-69db-7d82-bedf-868a512331d6" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/IntangibleAssetsandGoodwillIntangibleAssetsDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#IntangibleAssetsandGoodwillIntangibleAssetsDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/IntangibleAssetsandGoodwillIntangibleAssetsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_2921b5a0-0dba-451d-b345-25b0c70a243b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_11da2e07-92d5-4b7b-955c-fd774af130c9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_2921b5a0-0dba-451d-b345-25b0c70a243b" xlink:to="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_11da2e07-92d5-4b7b-955c-fd774af130c9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_00c29d5f-041d-4238-a1d8-5f5ed5456e1c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_11da2e07-92d5-4b7b-955c-fd774af130c9" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_00c29d5f-041d-4238-a1d8-5f5ed5456e1c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_00c29d5f-041d-4238-a1d8-5f5ed5456e1c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_00c29d5f-041d-4238-a1d8-5f5ed5456e1c" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_00c29d5f-041d-4238-a1d8-5f5ed5456e1c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_571dcb4e-116f-4379-bb13-b896f8d45afa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_00c29d5f-041d-4238-a1d8-5f5ed5456e1c" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_571dcb4e-116f-4379-bb13-b896f8d45afa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DevelopedTechnologyRightsMember_7f94c4ab-d4f2-431b-a989-a6eb51562cdf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DevelopedTechnologyRightsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_571dcb4e-116f-4379-bb13-b896f8d45afa" xlink:to="loc_us-gaap_DevelopedTechnologyRightsMember_7f94c4ab-d4f2-431b-a989-a6eb51562cdf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_08b848ec-db9c-476b-8a8e-b83509cef938" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_571dcb4e-116f-4379-bb13-b896f8d45afa" xlink:to="loc_us-gaap_CustomerRelationshipsMember_08b848ec-db9c-476b-8a8e-b83509cef938" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InProcessResearchAndDevelopmentMember_e3de3d6d-9a5a-45d9-810f-49b925b86592" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InProcessResearchAndDevelopmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_571dcb4e-116f-4379-bb13-b896f8d45afa" xlink:to="loc_us-gaap_InProcessResearchAndDevelopmentMember_e3de3d6d-9a5a-45d9-810f-49b925b86592" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherIntangibleAssetsMember_e17d5a65-f00c-4700-b7d1-8f249c2b0e85" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherIntangibleAssetsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_571dcb4e-116f-4379-bb13-b896f8d45afa" xlink:to="loc_us-gaap_OtherIntangibleAssetsMember_e17d5a65-f00c-4700-b7d1-8f249c2b0e85" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncompeteAgreementsMember_87491344-d828-451b-9821-1195f727fcea" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NoncompeteAgreementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_571dcb4e-116f-4379-bb13-b896f8d45afa" xlink:to="loc_us-gaap_NoncompeteAgreementsMember_87491344-d828-451b-9821-1195f727fcea" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsGross_151fe325-73ab-4257-b376-d9e0cce08d80" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_2921b5a0-0dba-451d-b345-25b0c70a243b" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsGross_151fe325-73ab-4257-b376-d9e0cce08d80" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_e584bc7a-7f20-47ac-9149-b8f447efda1f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_2921b5a0-0dba-451d-b345-25b0c70a243b" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_e584bc7a-7f20-47ac-9149-b8f447efda1f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNet_dd90013d-e6ee-4081-9fc3-3c0f5b7014cb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_2921b5a0-0dba-451d-b345-25b0c70a243b" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNet_dd90013d-e6ee-4081-9fc3-3c0f5b7014cb" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/LeasesNarrativeDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#LeasesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/LeasesNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionLineItems_7cf574c6-a60b-4a9e-bc8a-905205d11b23" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeLeaseDescriptionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionTable_cf29028e-d9d5-4a4f-bcb4-06c8778153e4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeLeaseDescriptionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_7cf574c6-a60b-4a9e-bc8a-905205d11b23" xlink:to="loc_us-gaap_LesseeLeaseDescriptionTable_cf29028e-d9d5-4a4f-bcb4-06c8778153e4" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_0d5fe5c3-f8c5-4fe6-9af8-b58a4cebbdda" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LesseeLeaseDescriptionTable_cf29028e-d9d5-4a4f-bcb4-06c8778153e4" xlink:to="loc_srt_RangeAxis_0d5fe5c3-f8c5-4fe6-9af8-b58a4cebbdda" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_0d5fe5c3-f8c5-4fe6-9af8-b58a4cebbdda_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_0d5fe5c3-f8c5-4fe6-9af8-b58a4cebbdda" xlink:to="loc_srt_RangeMember_0d5fe5c3-f8c5-4fe6-9af8-b58a4cebbdda_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_aeb90588-c01d-46b0-b19d-2ee83fa431f9" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_0d5fe5c3-f8c5-4fe6-9af8-b58a4cebbdda" xlink:to="loc_srt_RangeMember_aeb90588-c01d-46b0-b19d-2ee83fa431f9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_a2f9ffec-1b19-4e43-bf25-8920313c3d63" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_aeb90588-c01d-46b0-b19d-2ee83fa431f9" xlink:to="loc_srt_MinimumMember_a2f9ffec-1b19-4e43-bf25-8920313c3d63" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_6cd4b0c3-936f-461d-a9b5-39536d445b3b" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_aeb90588-c01d-46b0-b19d-2ee83fa431f9" xlink:to="loc_srt_MaximumMember_6cd4b0c3-936f-461d-a9b5-39536d445b3b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseContractualTermAxis_019c7d34-2423-70d7-83d1-8fe69392d79a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LeaseContractualTermAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LesseeLeaseDescriptionTable_cf29028e-d9d5-4a4f-bcb4-06c8778153e4" xlink:to="loc_us-gaap_LeaseContractualTermAxis_019c7d34-2423-70d7-83d1-8fe69392d79a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseContractualTermDomain_019c7d34-2423-70d7-83d1-8fe69392d79a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LeaseContractualTermDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LeaseContractualTermAxis_019c7d34-2423-70d7-83d1-8fe69392d79a" xlink:to="loc_us-gaap_LeaseContractualTermDomain_019c7d34-2423-70d7-83d1-8fe69392d79a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseContractualTermDomain_019c7d34-2423-7150-9500-51a0ff1e2fd9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LeaseContractualTermDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LeaseContractualTermAxis_019c7d34-2423-70d7-83d1-8fe69392d79a" xlink:to="loc_us-gaap_LeaseContractualTermDomain_019c7d34-2423-7150-9500-51a0ff1e2fd9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_LeasesCommencing2030Member_019c7d35-2591-7825-b3bb-b959787aa262" xlink:href="mgni-20251231.xsd#mgni_LeasesCommencing2030Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LeaseContractualTermDomain_019c7d34-2423-7150-9500-51a0ff1e2fd9" xlink:to="loc_mgni_LeasesCommencing2030Member_019c7d35-2591-7825-b3bb-b959787aa262" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseTermOfContract_8b8b319d-9a76-45a5-8bc4-96cd3dbd8807" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeOperatingLeaseTermOfContract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_7cf574c6-a60b-4a9e-bc8a-905205d11b23" xlink:to="loc_us-gaap_LesseeOperatingLeaseTermOfContract_8b8b319d-9a76-45a5-8bc4-96cd3dbd8807" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseWeightedAverageRemainingLeaseTerm1_eef27c11-035f-4f28-9e4d-3f3e9e365da9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseWeightedAverageRemainingLeaseTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_7cf574c6-a60b-4a9e-bc8a-905205d11b23" xlink:to="loc_us-gaap_OperatingLeaseWeightedAverageRemainingLeaseTerm1_eef27c11-035f-4f28-9e4d-3f3e9e365da9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent_64f2eea0-4228-4877-bbca-e66fad890d14" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_7cf574c6-a60b-4a9e-bc8a-905205d11b23" xlink:to="loc_us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent_64f2eea0-4228-4877-bbca-e66fad890d14" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseCost_019c7af4-983a-7391-8cb7-2ce8c9103455" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_7cf574c6-a60b-4a9e-bc8a-905205d11b23" xlink:to="loc_us-gaap_OperatingLeaseCost_019c7af4-983a-7391-8cb7-2ce8c9103455" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainLossOnTerminationOfLease_e4719011-0f61-42bf-a1e8-dcebfe877217" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GainLossOnTerminationOfLease"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_7cf574c6-a60b-4a9e-bc8a-905205d11b23" xlink:to="loc_us-gaap_GainLossOnTerminationOfLease_e4719011-0f61-42bf-a1e8-dcebfe877217" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseCost_a2a232e4-2fc0-4389-9d94-4f99297a6465" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LeaseCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_7cf574c6-a60b-4a9e-bc8a-905205d11b23" xlink:to="loc_us-gaap_LeaseCost_a2a232e4-2fc0-4389-9d94-4f99297a6465" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableLeaseCost_0ef7c674-6b8c-4c47-96cb-74e8d8cc975a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VariableLeaseCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_7cf574c6-a60b-4a9e-bc8a-905205d11b23" xlink:to="loc_us-gaap_VariableLeaseCost_0ef7c674-6b8c-4c47-96cb-74e8d8cc975a" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeasePayments_e9813e12-b36b-4caf-8804-b2b35d6a0e76" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeasePayments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_7cf574c6-a60b-4a9e-bc8a-905205d11b23" xlink:to="loc_us-gaap_OperatingLeasePayments_e9813e12-b36b-4caf-8804-b2b35d6a0e76" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubleaseIncome_091a4fcf-37f8-4611-a100-c18fd2c94589" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubleaseIncome"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_7cf574c6-a60b-4a9e-bc8a-905205d11b23" xlink:to="loc_us-gaap_SubleaseIncome_091a4fcf-37f8-4611-a100-c18fd2c94589" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceSheetAmount_e59dd849-cf2e-4324-8a1f-496a4ceb2c90" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceSheetAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_7cf574c6-a60b-4a9e-bc8a-905205d11b23" xlink:to="loc_us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceSheetAmount_e59dd849-cf2e-4324-8a1f-496a4ceb2c90" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseLeaseNotYetCommencedTermOfContract1_fda1d0bb-ec27-4eed-813d-624649f3fb3d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeOperatingLeaseLeaseNotYetCommencedTermOfContract1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_7cf574c6-a60b-4a9e-bc8a-905205d11b23" xlink:to="loc_us-gaap_LesseeOperatingLeaseLeaseNotYetCommencedTermOfContract1_fda1d0bb-ec27-4eed-813d-624649f3fb3d" xlink:type="arc" order="10"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/LeasesScheduleofRightOfUseAssetsbyGeographicalRegionDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#LeasesScheduleofRightOfUseAssetsbyGeographicalRegionDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/LeasesScheduleofRightOfUseAssetsbyGeographicalRegionDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionLineItems_89069e4d-ee4b-48ce-8934-a1b592a5e675" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeLeaseDescriptionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionTable_ff2587ee-e62e-453c-b5da-f5663fc66baa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LesseeLeaseDescriptionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_89069e4d-ee4b-48ce-8934-a1b592a5e675" xlink:to="loc_us-gaap_LesseeLeaseDescriptionTable_ff2587ee-e62e-453c-b5da-f5663fc66baa" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_e0175ea6-03d2-407f-81d6-90384f5bdc3b" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LesseeLeaseDescriptionTable_ff2587ee-e62e-453c-b5da-f5663fc66baa" xlink:to="loc_srt_StatementGeographicalAxis_e0175ea6-03d2-407f-81d6-90384f5bdc3b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_e0175ea6-03d2-407f-81d6-90384f5bdc3b_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_e0175ea6-03d2-407f-81d6-90384f5bdc3b" xlink:to="loc_srt_SegmentGeographicalDomain_e0175ea6-03d2-407f-81d6-90384f5bdc3b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_aa45fe4e-cba4-467f-8b36-c8bf707de134" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_e0175ea6-03d2-407f-81d6-90384f5bdc3b" xlink:to="loc_srt_SegmentGeographicalDomain_aa45fe4e-cba4-467f-8b36-c8bf707de134" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_9f9a5bd7-4642-427a-bd1a-25cacfd6842a" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_aa45fe4e-cba4-467f-8b36-c8bf707de134" xlink:to="loc_country_US_9f9a5bd7-4642-427a-bd1a-25cacfd6842a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NonUsMember_da228a5d-a425-49d8-b91a-41fd1c887bde" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NonUsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_aa45fe4e-cba4-467f-8b36-c8bf707de134" xlink:to="loc_us-gaap_NonUsMember_da228a5d-a425-49d8-b91a-41fd1c887bde" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseRightOfUseAsset_cc0de46e-976d-468a-ad0b-20c555e6e732" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseRightOfUseAsset"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_89069e4d-ee4b-48ce-8934-a1b592a5e675" xlink:to="loc_us-gaap_OperatingLeaseRightOfUseAsset_cc0de46e-976d-468a-ad0b-20c555e6e732" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/DebtScheduleofLongTermDebtDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#DebtScheduleofLongTermDebtDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/DebtScheduleofLongTermDebtDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_126a5730-7dd4-452c-ab55-b98d95fdcfee" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_0290a6c1-6c1c-43bb-8952-8493adc797f5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_126a5730-7dd4-452c-ab55-b98d95fdcfee" xlink:to="loc_us-gaap_DebtInstrumentTable_0290a6c1-6c1c-43bb-8952-8493adc797f5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_fb4d7247-f949-411a-b4ac-1b68143a77aa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_0290a6c1-6c1c-43bb-8952-8493adc797f5" xlink:to="loc_us-gaap_DebtInstrumentAxis_fb4d7247-f949-411a-b4ac-1b68143a77aa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_fb4d7247-f949-411a-b4ac-1b68143a77aa_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_fb4d7247-f949-411a-b4ac-1b68143a77aa" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_fb4d7247-f949-411a-b4ac-1b68143a77aa_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_dba1ead3-1544-487a-a3c7-6e5653229fde" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_fb4d7247-f949-411a-b4ac-1b68143a77aa" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_dba1ead3-1544-487a-a3c7-6e5653229fde" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ConvertibleSeniorNoteMember_d76f44be-0253-4c40-a213-c22937626b05" xlink:href="mgni-20251231.xsd#mgni_ConvertibleSeniorNoteMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_dba1ead3-1544-487a-a3c7-6e5653229fde" xlink:to="loc_mgni_ConvertibleSeniorNoteMember_d76f44be-0253-4c40-a213-c22937626b05" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_CreditAgreement2024Member_019b7ce9-5264-7ab4-bfb8-7cc686eb7296" xlink:href="mgni-20251231.xsd#mgni_CreditAgreement2024Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_dba1ead3-1544-487a-a3c7-6e5653229fde" xlink:to="loc_mgni_CreditAgreement2024Member_019b7ce9-5264-7ab4-bfb8-7cc686eb7296" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_65cd8f8f-7b66-4439-a5e4-7bce0bdba4d0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_0290a6c1-6c1c-43bb-8952-8493adc797f5" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_65cd8f8f-7b66-4439-a5e4-7bce0bdba4d0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_65cd8f8f-7b66-4439-a5e4-7bce0bdba4d0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_65cd8f8f-7b66-4439-a5e4-7bce0bdba4d0" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_65cd8f8f-7b66-4439-a5e4-7bce0bdba4d0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_cb989c91-979d-42d8-93de-583f569f3910" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_65cd8f8f-7b66-4439-a5e4-7bce0bdba4d0" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_cb989c91-979d-42d8-93de-583f569f3910" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtMember_425811fd-3563-42bd-833d-27fcefe7f00d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConvertibleDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_cb989c91-979d-42d8-93de-583f569f3910" xlink:to="loc_us-gaap_ConvertibleDebtMember_425811fd-3563-42bd-833d-27fcefe7f00d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_09dbddd8-60b3-4f39-a5f2-78983fabbe26" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_cb989c91-979d-42d8-93de-583f569f3910" xlink:to="loc_us-gaap_LineOfCreditMember_09dbddd8-60b3-4f39-a5f2-78983fabbe26" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_489b2f3d-8cb2-42e0-857a-47dfe1c15fb6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_0290a6c1-6c1c-43bb-8952-8493adc797f5" xlink:to="loc_us-gaap_CreditFacilityAxis_489b2f3d-8cb2-42e0-857a-47dfe1c15fb6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_489b2f3d-8cb2-42e0-857a-47dfe1c15fb6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_489b2f3d-8cb2-42e0-857a-47dfe1c15fb6" xlink:to="loc_us-gaap_CreditFacilityDomain_489b2f3d-8cb2-42e0-857a-47dfe1c15fb6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_032ee46e-deca-4a9b-8207-ddcf9633bef9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_489b2f3d-8cb2-42e0-857a-47dfe1c15fb6" xlink:to="loc_us-gaap_CreditFacilityDomain_032ee46e-deca-4a9b-8207-ddcf9633bef9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_3b9dbbd1-7d61-47ec-a8d5-9546312e76e1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_032ee46e-deca-4a9b-8207-ddcf9633bef9" xlink:to="loc_us-gaap_SecuredDebtMember_3b9dbbd1-7d61-47ec-a8d5-9546312e76e1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_7e5bef6d-85f3-4010-955b-d4bc54c658e8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_126a5730-7dd4-452c-ab55-b98d95fdcfee" xlink:to="loc_us-gaap_DebtInstrumentCarryingAmount_7e5bef6d-85f3-4010-955b-d4bc54c658e8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet_545e28c2-9ccb-47ff-92c2-6a6fdbcc13cd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_126a5730-7dd4-452c-ab55-b98d95fdcfee" xlink:to="loc_us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet_545e28c2-9ccb-47ff-92c2-6a6fdbcc13cd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtCurrent_2898e219-8e96-4d8a-a1dd-b8647383503b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_126a5730-7dd4-452c-ab55-b98d95fdcfee" xlink:to="loc_us-gaap_DebtCurrent_2898e219-8e96-4d8a-a1dd-b8647383503b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtNoncurrent_c1c71284-d055-4ad0-b90b-9eba360d88f3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_126a5730-7dd4-452c-ab55-b98d95fdcfee" xlink:to="loc_us-gaap_LongTermDebtNoncurrent_c1c71284-d055-4ad0-b90b-9eba360d88f3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebt_fb9d7b37-3075-4045-ad65-cc0c60cb5be1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_126a5730-7dd4-452c-ab55-b98d95fdcfee" xlink:to="loc_us-gaap_LongTermDebt_fb9d7b37-3075-4045-ad65-cc0c60cb5be1" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/DebtConvertibleSeniorNotesandCappedCallTransactionsNarrativeDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#DebtConvertibleSeniorNotesandCappedCallTransactionsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/DebtConvertibleSeniorNotesandCappedCallTransactionsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019b7d26-d6ed-7b3e-bf85-57c283ce4663" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_5ffdd79d-3b35-4bc6-b7f3-3af73ef1fde7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019b7d26-d6ed-7b3e-bf85-57c283ce4663" xlink:to="loc_us-gaap_DebtInstrumentTable_5ffdd79d-3b35-4bc6-b7f3-3af73ef1fde7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019b7d26-d6ed-7dbd-a44f-94d6f655d27b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_5ffdd79d-3b35-4bc6-b7f3-3af73ef1fde7" xlink:to="loc_us-gaap_DebtInstrumentAxis_019b7d26-d6ed-7dbd-a44f-94d6f655d27b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019b7d26-d6ed-7dbd-a44f-94d6f655d27b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019b7d26-d6ed-7dbd-a44f-94d6f655d27b" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019b7d26-d6ed-7dbd-a44f-94d6f655d27b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019b7d26-d6ed-79b4-9abe-a34dc6960b2a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019b7d26-d6ed-7dbd-a44f-94d6f655d27b" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019b7d26-d6ed-79b4-9abe-a34dc6960b2a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ConvertibleSeniorNoteMember_019b7d26-d6ed-741d-adec-1eb2ff7985ec" xlink:href="mgni-20251231.xsd#mgni_ConvertibleSeniorNoteMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019b7d26-d6ed-79b4-9abe-a34dc6960b2a" xlink:to="loc_mgni_ConvertibleSeniorNoteMember_019b7d26-d6ed-741d-adec-1eb2ff7985ec" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019b7d26-d6ed-7a80-85ce-7b756e45eeb5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_5ffdd79d-3b35-4bc6-b7f3-3af73ef1fde7" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019b7d26-d6ed-7a80-85ce-7b756e45eeb5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019b7d26-d6ed-7a80-85ce-7b756e45eeb5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019b7d26-d6ed-7a80-85ce-7b756e45eeb5" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019b7d26-d6ed-7a80-85ce-7b756e45eeb5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019b7d26-d6ed-7287-bc97-124460ccf392" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019b7d26-d6ed-7a80-85ce-7b756e45eeb5" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019b7d26-d6ed-7287-bc97-124460ccf392" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtMember_019b7d26-d6ed-7bea-a88f-a70413983bde" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConvertibleDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019b7d26-d6ed-7287-bc97-124460ccf392" xlink:to="loc_us-gaap_ConvertibleDebtMember_019b7d26-d6ed-7bea-a88f-a70413983bde" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_DebtInstrumentConversionTermAxis_019b7d26-d6ed-7afd-8395-07d4b21146f8" xlink:href="mgni-20251231.xsd#mgni_DebtInstrumentConversionTermAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_5ffdd79d-3b35-4bc6-b7f3-3af73ef1fde7" xlink:to="loc_mgni_DebtInstrumentConversionTermAxis_019b7d26-d6ed-7afd-8395-07d4b21146f8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_DebtInstrumentConversionTermDomain_019b7d26-d6ed-7afd-8395-07d4b21146f8_default" xlink:href="mgni-20251231.xsd#mgni_DebtInstrumentConversionTermDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_mgni_DebtInstrumentConversionTermAxis_019b7d26-d6ed-7afd-8395-07d4b21146f8" xlink:to="loc_mgni_DebtInstrumentConversionTermDomain_019b7d26-d6ed-7afd-8395-07d4b21146f8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_DebtInstrumentConversionTermDomain_019b7d26-d6ed-7390-bfd9-083ecec0cb4b" xlink:href="mgni-20251231.xsd#mgni_DebtInstrumentConversionTermDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_mgni_DebtInstrumentConversionTermAxis_019b7d26-d6ed-7afd-8395-07d4b21146f8" xlink:to="loc_mgni_DebtInstrumentConversionTermDomain_019b7d26-d6ed-7390-bfd9-083ecec0cb4b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ConversionTermIMember_019b7d26-d6ed-7ff9-9469-c33f6b9e29be" xlink:href="mgni-20251231.xsd#mgni_ConversionTermIMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_DebtInstrumentConversionTermDomain_019b7d26-d6ed-7390-bfd9-083ecec0cb4b" xlink:to="loc_mgni_ConversionTermIMember_019b7d26-d6ed-7ff9-9469-c33f6b9e29be" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ConversionTermIiMember_019b7d26-d6ed-7eaa-b259-fab9eb8987b5" xlink:href="mgni-20251231.xsd#mgni_ConversionTermIiMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_DebtInstrumentConversionTermDomain_019b7d26-d6ed-7390-bfd9-083ecec0cb4b" xlink:to="loc_mgni_ConversionTermIiMember_019b7d26-d6ed-7eaa-b259-fab9eb8987b5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ConversionTermVMember_019b7d26-d6ed-74b6-885f-6a4cff2ae825" xlink:href="mgni-20251231.xsd#mgni_ConversionTermVMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_DebtInstrumentConversionTermDomain_019b7d26-d6ed-7390-bfd9-083ecec0cb4b" xlink:to="loc_mgni_ConversionTermVMember_019b7d26-d6ed-74b6-885f-6a4cff2ae825" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ConversionTermIvMember_019b7d26-d6ed-7e8c-acb8-9924bb07abfc" xlink:href="mgni-20251231.xsd#mgni_ConversionTermIvMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_DebtInstrumentConversionTermDomain_019b7d26-d6ed-7390-bfd9-083ecec0cb4b" xlink:to="loc_mgni_ConversionTermIvMember_019b7d26-d6ed-7e8c-acb8-9924bb07abfc" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019b7d27-3149-736d-9827-83ba0f7c2bff" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_5ffdd79d-3b35-4bc6-b7f3-3af73ef1fde7" xlink:to="loc_srt_RangeAxis_019b7d27-3149-736d-9827-83ba0f7c2bff" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019b7d27-3149-736d-9827-83ba0f7c2bff_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019b7d27-3149-736d-9827-83ba0f7c2bff" xlink:to="loc_srt_RangeMember_019b7d27-3149-736d-9827-83ba0f7c2bff_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019b7d27-3149-7175-a8d5-2922ee4fd396" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019b7d27-3149-736d-9827-83ba0f7c2bff" xlink:to="loc_srt_RangeMember_019b7d27-3149-7175-a8d5-2922ee4fd396" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019b7d27-3149-76f1-9499-d472aafdb3c8" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019b7d27-3149-7175-a8d5-2922ee4fd396" xlink:to="loc_srt_MaximumMember_019b7d27-3149-76f1-9499-d472aafdb3c8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019b7d27-3149-78a9-971c-a16ba145d00a" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019b7d27-3149-7175-a8d5-2922ee4fd396" xlink:to="loc_srt_MinimumMember_019b7d27-3149-78a9-971c-a16ba145d00a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_019b7d26-d6ed-7ec8-a54b-9fbc123a8e37" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019b7d26-d6ed-7b3e-bf85-57c283ce4663" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_019b7d26-d6ed-7ec8-a54b-9fbc123a8e37" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019b7d26-d6ed-7833-b5e0-02d582896cf9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019b7d26-d6ed-7b3e-bf85-57c283ce4663" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019b7d26-d6ed-7833-b5e0-02d582896cf9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_DebtInstrumentOverAllotmentOptions_019b7d26-d6ed-7c89-8d8a-86181063e5ef" xlink:href="mgni-20251231.xsd#mgni_DebtInstrumentOverAllotmentOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019b7d26-d6ed-7b3e-bf85-57c283ce4663" xlink:to="loc_mgni_DebtInstrumentOverAllotmentOptions_019b7d26-d6ed-7c89-8d8a-86181063e5ef" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleThresholdPercentageOfStockPriceTrigger_019b7d26-d6ed-77b4-b27a-e14aea5a74ca" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentConvertibleThresholdPercentageOfStockPriceTrigger"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019b7d26-d6ed-7b3e-bf85-57c283ce4663" xlink:to="loc_us-gaap_DebtInstrumentConvertibleThresholdPercentageOfStockPriceTrigger_019b7d26-d6ed-77b4-b27a-e14aea5a74ca" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleThresholdConsecutiveTradingDays1_019b7d26-d6ed-71e6-95ac-ac1c2199b243" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentConvertibleThresholdConsecutiveTradingDays1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019b7d26-d6ed-7b3e-bf85-57c283ce4663" xlink:to="loc_us-gaap_DebtInstrumentConvertibleThresholdConsecutiveTradingDays1_019b7d26-d6ed-71e6-95ac-ac1c2199b243" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleThresholdTradingDays_019b7d26-d6ed-7693-96ea-4535aa0ad0bb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentConvertibleThresholdTradingDays"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019b7d26-d6ed-7b3e-bf85-57c283ce4663" xlink:to="loc_us-gaap_DebtInstrumentConvertibleThresholdTradingDays_019b7d26-d6ed-7693-96ea-4535aa0ad0bb" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_DebtInstrumentRedemptionMakeWholeFundamentalChangePeriod_019b7d27-3149-7e43-be71-7b5b9b04e008" xlink:href="mgni-20251231.xsd#mgni_DebtInstrumentRedemptionMakeWholeFundamentalChangePeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019b7d26-d6ed-7b3e-bf85-57c283ce4663" xlink:to="loc_mgni_DebtInstrumentRedemptionMakeWholeFundamentalChangePeriod_019b7d27-3149-7e43-be71-7b5b9b04e008" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_PercentageOfOutstandingBalanceHoldersAbleToCallDebtInTheEventOfDefault_019b7d27-3149-7fb7-91c4-a2fc623cd329" xlink:href="mgni-20251231.xsd#mgni_PercentageOfOutstandingBalanceHoldersAbleToCallDebtInTheEventOfDefault"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019b7d26-d6ed-7b3e-bf85-57c283ce4663" xlink:to="loc_mgni_PercentageOfOutstandingBalanceHoldersAbleToCallDebtInTheEventOfDefault_019b7d27-3149-7fb7-91c4-a2fc623cd329" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentConvertibleConversionRatio1_019b8ead-d4ac-76b1-9a8d-865dbf665388" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentConvertibleConversionRatio1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019b7d26-d6ed-7b3e-bf85-57c283ce4663" xlink:to="loc_us-gaap_DebtInstrumentConvertibleConversionRatio1_019b8ead-d4ac-76b1-9a8d-865dbf665388" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_DebtInstrumentCappedCallConversionPrice_019b7d27-3149-7bfa-bc9b-eb7180240a2e" xlink:href="mgni-20251231.xsd#mgni_DebtInstrumentCappedCallConversionPrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019b7d26-d6ed-7b3e-bf85-57c283ce4663" xlink:to="loc_mgni_DebtInstrumentCappedCallConversionPrice_019b7d27-3149-7bfa-bc9b-eb7180240a2e" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_DebtInstrumentsCappedCallsTransactionCosts_019b7d26-d6ed-73a4-85b0-bd77fb2f81e4" xlink:href="mgni-20251231.xsd#mgni_DebtInstrumentsCappedCallsTransactionCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019b7d26-d6ed-7b3e-bf85-57c283ce4663" xlink:to="loc_mgni_DebtInstrumentsCappedCallsTransactionCosts_019b7d26-d6ed-73a4-85b0-bd77fb2f81e4" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredFinanceCostsGross_019b7d27-3149-777f-b029-7cee50f9507c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredFinanceCostsGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019b7d26-d6ed-7b3e-bf85-57c283ce4663" xlink:to="loc_us-gaap_DeferredFinanceCostsGross_019b7d27-3149-777f-b029-7cee50f9507c" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RepaymentsOfConvertibleDebt_019b7d27-3149-7c11-817c-08fe3d9d333b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RepaymentsOfConvertibleDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019b7d26-d6ed-7b3e-bf85-57c283ce4663" xlink:to="loc_us-gaap_RepaymentsOfConvertibleDebt_019b7d27-3149-7c11-817c-08fe3d9d333b" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainsLossesOnExtinguishmentOfDebt_019b7d27-3149-7a61-a580-55cd30d9a878" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GainsLossesOnExtinguishmentOfDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019b7d26-d6ed-7b3e-bf85-57c283ce4663" xlink:to="loc_us-gaap_GainsLossesOnExtinguishmentOfDebt_019b7d27-3149-7a61-a580-55cd30d9a878" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_DebtInstrumentRepurchasedDuringPeriodFaceAmount_019b7d27-3149-72a9-b05d-8607d45851e2" xlink:href="mgni-20251231.xsd#mgni_DebtInstrumentRepurchasedDuringPeriodFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019b7d26-d6ed-7b3e-bf85-57c283ce4663" xlink:to="loc_mgni_DebtInstrumentRepurchasedDuringPeriodFaceAmount_019b7d27-3149-72a9-b05d-8607d45851e2" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WriteOffOfDeferredDebtIssuanceCost_019b7d27-3149-7d5a-bf44-f5b36893fb7d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WriteOffOfDeferredDebtIssuanceCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019b7d26-d6ed-7b3e-bf85-57c283ce4663" xlink:to="loc_us-gaap_WriteOffOfDeferredDebtIssuanceCost_019b7d27-3149-7d5a-bf44-f5b36893fb7d" xlink:type="arc" order="15"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/DebtInterestExpenseRelatedtotheConvertibleSeniorNotesDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#DebtInterestExpenseRelatedtotheConvertibleSeniorNotesDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/DebtInterestExpenseRelatedtotheConvertibleSeniorNotesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_bad7c3d9-f4fb-415e-a2c3-c1d548670995" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_e4be0bb0-914a-4f4c-8f02-92eb2ccdc16b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_bad7c3d9-f4fb-415e-a2c3-c1d548670995" xlink:to="loc_us-gaap_DebtInstrumentTable_e4be0bb0-914a-4f4c-8f02-92eb2ccdc16b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_37e1c116-c149-4f80-bb84-501adba37732" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_e4be0bb0-914a-4f4c-8f02-92eb2ccdc16b" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_37e1c116-c149-4f80-bb84-501adba37732" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_37e1c116-c149-4f80-bb84-501adba37732_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_37e1c116-c149-4f80-bb84-501adba37732" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_37e1c116-c149-4f80-bb84-501adba37732_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_cc33f38f-a335-4ef5-a443-ea8aead6c6f4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_37e1c116-c149-4f80-bb84-501adba37732" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_cc33f38f-a335-4ef5-a443-ea8aead6c6f4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConvertibleDebtMember_56e98824-6d14-499a-bc0f-2b6fd7f83640" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConvertibleDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_cc33f38f-a335-4ef5-a443-ea8aead6c6f4" xlink:to="loc_us-gaap_ConvertibleDebtMember_56e98824-6d14-499a-bc0f-2b6fd7f83640" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_c3d86369-812b-4ae3-97ba-9c0dca8a16d8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_e4be0bb0-914a-4f4c-8f02-92eb2ccdc16b" xlink:to="loc_us-gaap_DebtInstrumentAxis_c3d86369-812b-4ae3-97ba-9c0dca8a16d8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_c3d86369-812b-4ae3-97ba-9c0dca8a16d8_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_c3d86369-812b-4ae3-97ba-9c0dca8a16d8" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_c3d86369-812b-4ae3-97ba-9c0dca8a16d8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_d8485e17-79c7-4092-951b-6bfb8e471e22" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_c3d86369-812b-4ae3-97ba-9c0dca8a16d8" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_d8485e17-79c7-4092-951b-6bfb8e471e22" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ConvertibleSeniorNoteMember_578e65b9-33df-407e-b4f9-39fc5f560741" xlink:href="mgni-20251231.xsd#mgni_ConvertibleSeniorNoteMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_d8485e17-79c7-4092-951b-6bfb8e471e22" xlink:to="loc_mgni_ConvertibleSeniorNoteMember_578e65b9-33df-407e-b4f9-39fc5f560741" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseDebt_0413fde5-c397-4434-843e-3a77424af952" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestExpenseDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_bad7c3d9-f4fb-415e-a2c3-c1d548670995" xlink:to="loc_us-gaap_InterestExpenseDebt_0413fde5-c397-4434-843e-3a77424af952" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfFinancingCosts_9087315b-e932-46ba-9075-cd6fa5c2b612" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AmortizationOfFinancingCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_bad7c3d9-f4fb-415e-a2c3-c1d548670995" xlink:to="loc_us-gaap_AmortizationOfFinancingCosts_9087315b-e932-46ba-9075-cd6fa5c2b612" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpense_23b3bedb-25f0-468d-bef7-8e242e977686" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_bad7c3d9-f4fb-415e-a2c3-c1d548670995" xlink:to="loc_us-gaap_InterestExpense_23b3bedb-25f0-468d-bef7-8e242e977686" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_5007a613-e308-4501-aba9-084d08aa00ed" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentInterestRateEffectivePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_bad7c3d9-f4fb-415e-a2c3-c1d548670995" xlink:to="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_5007a613-e308-4501-aba9-084d08aa00ed" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/DebtCreditAgreementNarrativeDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#DebtCreditAgreementNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/DebtCreditAgreementNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityLineItems_019b7d8c-aa32-7808-bc4a-1a8bb2f68c2e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityTable_0bf96d59-b5f5-4e08-ba69-64d1e16fa8be" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019b7d8c-aa32-7808-bc4a-1a8bb2f68c2e" xlink:to="loc_us-gaap_LineOfCreditFacilityTable_0bf96d59-b5f5-4e08-ba69-64d1e16fa8be" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_019b7d8c-aa32-7ea2-b793-238ae0a3d6e3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_0bf96d59-b5f5-4e08-ba69-64d1e16fa8be" xlink:to="loc_us-gaap_CreditFacilityAxis_019b7d8c-aa32-7ea2-b793-238ae0a3d6e3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019b7d8c-aa32-7ea2-b793-238ae0a3d6e3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_019b7d8c-aa32-7ea2-b793-238ae0a3d6e3" xlink:to="loc_us-gaap_CreditFacilityDomain_019b7d8c-aa32-7ea2-b793-238ae0a3d6e3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019b7d8c-aa32-79f3-8143-dfd5a0ac7986" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_019b7d8c-aa32-7ea2-b793-238ae0a3d6e3" xlink:to="loc_us-gaap_CreditFacilityDomain_019b7d8c-aa32-79f3-8143-dfd5a0ac7986" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_019b7d8c-aa32-7398-8ff9-e67912909ccb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019b7d8c-aa32-79f3-8143-dfd5a0ac7986" xlink:to="loc_us-gaap_SecuredDebtMember_019b7d8c-aa32-7398-8ff9-e67912909ccb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_019b7d8c-aa32-7dcb-bd42-2fec322e07c5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevolvingCreditFacilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019b7d8c-aa32-79f3-8143-dfd5a0ac7986" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_019b7d8c-aa32-7dcb-bd42-2fec322e07c5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LetterOfCreditMember_019b7d8c-ed8e-710b-b151-d82629f8fcb6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LetterOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019b7d8c-aa32-79f3-8143-dfd5a0ac7986" xlink:to="loc_us-gaap_LetterOfCreditMember_019b7d8c-ed8e-710b-b151-d82629f8fcb6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019b7d8c-aa32-70c4-9234-10039b0c68ce" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_0bf96d59-b5f5-4e08-ba69-64d1e16fa8be" xlink:to="loc_us-gaap_DebtInstrumentAxis_019b7d8c-aa32-70c4-9234-10039b0c68ce" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019b7d8c-aa32-70c4-9234-10039b0c68ce_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019b7d8c-aa32-70c4-9234-10039b0c68ce" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019b7d8c-aa32-70c4-9234-10039b0c68ce_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019b7d8c-aa32-7c4d-b847-49114f7ea429" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019b7d8c-aa32-70c4-9234-10039b0c68ce" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019b7d8c-aa32-7c4d-b847-49114f7ea429" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_CreditAgreement2021Member_019b7d8c-aa32-70ed-8502-bc7df7c6ed63" xlink:href="mgni-20251231.xsd#mgni_CreditAgreement2021Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019b7d8c-aa32-7c4d-b847-49114f7ea429" xlink:to="loc_mgni_CreditAgreement2021Member_019b7d8c-aa32-70ed-8502-bc7df7c6ed63" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_CreditAgreement2024Member_019b7d8c-aa32-7dc6-8590-ce62f7bcb5d1" xlink:href="mgni-20251231.xsd#mgni_CreditAgreement2024Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019b7d8c-aa32-7c4d-b847-49114f7ea429" xlink:to="loc_mgni_CreditAgreement2024Member_019b7d8c-aa32-7dc6-8590-ce62f7bcb5d1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_CreditAgreements2021And2024Member_019b7d94-633e-7772-9521-b5b007601d75" xlink:href="mgni-20251231.xsd#mgni_CreditAgreements2021And2024Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019b7d8c-aa32-7c4d-b847-49114f7ea429" xlink:to="loc_mgni_CreditAgreements2021And2024Member_019b7d94-633e-7772-9521-b5b007601d75" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_CreditAgreement2024DebtRepricingMember_019b7d94-633e-7064-b1fc-53b4211c88b3" xlink:href="mgni-20251231.xsd#mgni_CreditAgreement2024DebtRepricingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019b7d8c-aa32-7c4d-b847-49114f7ea429" xlink:to="loc_mgni_CreditAgreement2024DebtRepricingMember_019b7d94-633e-7064-b1fc-53b4211c88b3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019b7d8c-aa32-7fb0-aafd-1cdeb665e00d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_0bf96d59-b5f5-4e08-ba69-64d1e16fa8be" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019b7d8c-aa32-7fb0-aafd-1cdeb665e00d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019b7d8c-aa32-7fb0-aafd-1cdeb665e00d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019b7d8c-aa32-7fb0-aafd-1cdeb665e00d" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019b7d8c-aa32-7fb0-aafd-1cdeb665e00d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019b7d8c-aa32-794a-8f2c-5874c3887a90" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019b7d8c-aa32-7fb0-aafd-1cdeb665e00d" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019b7d8c-aa32-794a-8f2c-5874c3887a90" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_019b7d8c-aa32-776a-b806-6975e80e198e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019b7d8c-aa32-794a-8f2c-5874c3887a90" xlink:to="loc_us-gaap_LineOfCreditMember_019b7d8c-aa32-776a-b806-6975e80e198e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateAxis_019b7d8c-aa32-7785-b676-be4435c59823" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VariableRateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_0bf96d59-b5f5-4e08-ba69-64d1e16fa8be" xlink:to="loc_us-gaap_VariableRateAxis_019b7d8c-aa32-7785-b676-be4435c59823" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_019b7d8c-aa32-7785-b676-be4435c59823_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VariableRateAxis_019b7d8c-aa32-7785-b676-be4435c59823" xlink:to="loc_us-gaap_VariableRateDomain_019b7d8c-aa32-7785-b676-be4435c59823_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VariableRateDomain_019b7d8c-aa32-7d89-a757-9a4a49f2b674" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VariableRateDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VariableRateAxis_019b7d8c-aa32-7785-b676-be4435c59823" xlink:to="loc_us-gaap_VariableRateDomain_019b7d8c-aa32-7d89-a757-9a4a49f2b674" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredOvernightFinancingRateSofrMember_019b7d8c-aa32-77c6-829d-8c6aaf60d627" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SecuredOvernightFinancingRateSofrMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_019b7d8c-aa32-7d89-a757-9a4a49f2b674" xlink:to="loc_us-gaap_SecuredOvernightFinancingRateSofrMember_019b7d8c-aa32-77c6-829d-8c6aaf60d627" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_AlternateBaseRateMember_019b7d8c-aa32-766b-9e0e-d019abbf792e" xlink:href="mgni-20251231.xsd#mgni_AlternateBaseRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VariableRateDomain_019b7d8c-aa32-7d89-a757-9a4a49f2b674" xlink:to="loc_mgni_AlternateBaseRateMember_019b7d8c-aa32-766b-9e0e-d019abbf792e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019b7d8c-aa32-799c-9147-dac211ae11e2" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_0bf96d59-b5f5-4e08-ba69-64d1e16fa8be" xlink:to="loc_srt_RangeAxis_019b7d8c-aa32-799c-9147-dac211ae11e2" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019b7d8c-aa32-799c-9147-dac211ae11e2_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019b7d8c-aa32-799c-9147-dac211ae11e2" xlink:to="loc_srt_RangeMember_019b7d8c-aa32-799c-9147-dac211ae11e2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019b7d8c-aa32-7999-bccf-3221ba3148e9" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019b7d8c-aa32-799c-9147-dac211ae11e2" xlink:to="loc_srt_RangeMember_019b7d8c-aa32-7999-bccf-3221ba3148e9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019b7d8c-aa32-78a1-b3bc-606a06ab8f96" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019b7d8c-aa32-7999-bccf-3221ba3148e9" xlink:to="loc_srt_MinimumMember_019b7d8c-aa32-78a1-b3bc-606a06ab8f96" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019b7d8c-aa32-7cb4-be8a-33ff6ff79eef" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019b7d8c-aa32-7999-bccf-3221ba3148e9" xlink:to="loc_srt_MaximumMember_019b7d8c-aa32-7cb4-be8a-33ff6ff79eef" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_DebtIssuanceCostsAxis_019b7d94-633e-7cab-a1c5-aa528ba29af4" xlink:href="mgni-20251231.xsd#mgni_DebtIssuanceCostsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_0bf96d59-b5f5-4e08-ba69-64d1e16fa8be" xlink:to="loc_mgni_DebtIssuanceCostsAxis_019b7d94-633e-7cab-a1c5-aa528ba29af4" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_DebtIssuanceCostsDomain_019b7d94-633e-7cab-a1c5-aa528ba29af4_default" xlink:href="mgni-20251231.xsd#mgni_DebtIssuanceCostsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_mgni_DebtIssuanceCostsAxis_019b7d94-633e-7cab-a1c5-aa528ba29af4" xlink:to="loc_mgni_DebtIssuanceCostsDomain_019b7d94-633e-7cab-a1c5-aa528ba29af4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_DebtIssuanceCostsDomain_019b7d94-633e-7652-af79-0648a26bfae2" xlink:href="mgni-20251231.xsd#mgni_DebtIssuanceCostsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_mgni_DebtIssuanceCostsAxis_019b7d94-633e-7cab-a1c5-aa528ba29af4" xlink:to="loc_mgni_DebtIssuanceCostsDomain_019b7d94-633e-7652-af79-0648a26bfae2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_DebtDiscountMember_019b7d94-633e-78ae-9df9-1256ad3959fc" xlink:href="mgni-20251231.xsd#mgni_DebtDiscountMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_DebtIssuanceCostsDomain_019b7d94-633e-7652-af79-0648a26bfae2" xlink:to="loc_mgni_DebtDiscountMember_019b7d94-633e-78ae-9df9-1256ad3959fc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_DeferredFinancingCostsMember_019b7d94-633e-78a0-9760-473d0e536c06" xlink:href="mgni-20251231.xsd#mgni_DeferredFinancingCostsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_DebtIssuanceCostsDomain_019b7d94-633e-7652-af79-0648a26bfae2" xlink:to="loc_mgni_DeferredFinancingCostsMember_019b7d94-633e-78a0-9760-473d0e536c06" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_DebtRefinanceCostMember_019b7d94-633e-7f48-9334-5645cfab3d17" xlink:href="mgni-20251231.xsd#mgni_DebtRefinanceCostMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_DebtIssuanceCostsDomain_019b7d94-633e-7652-af79-0648a26bfae2" xlink:to="loc_mgni_DebtRefinanceCostMember_019b7d94-633e-7f48-9334-5645cfab3d17" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualAxis_019b7d94-633e-77b9-9364-f7897936e4d7" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_TitleOfIndividualAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LineOfCreditFacilityTable_0bf96d59-b5f5-4e08-ba69-64d1e16fa8be" xlink:to="loc_srt_TitleOfIndividualAxis_019b7d94-633e-77b9-9364-f7897936e4d7" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019b7d94-633e-77b9-9364-f7897936e4d7_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_TitleOfIndividualAxis_019b7d94-633e-77b9-9364-f7897936e4d7" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019b7d94-633e-77b9-9364-f7897936e4d7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019b7d94-633e-7c91-90dd-862509d7913a" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_TitleOfIndividualAxis_019b7d94-633e-77b9-9364-f7897936e4d7" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019b7d94-633e-7c91-90dd-862509d7913a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ExistingLendersMember_019b7d94-633e-7274-af75-5a03fb94d69f" xlink:href="mgni-20251231.xsd#mgni_ExistingLendersMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019b7d94-633e-7c91-90dd-862509d7913a" xlink:to="loc_mgni_ExistingLendersMember_019b7d94-633e-7274-af75-5a03fb94d69f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_NewLendersMember_019b7d94-633e-7200-9667-69883ce2f280" xlink:href="mgni-20251231.xsd#mgni_NewLendersMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019b7d94-633e-7c91-90dd-862509d7913a" xlink:to="loc_mgni_NewLendersMember_019b7d94-633e-7200-9667-69883ce2f280" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_019b7d8c-aa32-7812-afce-42515f5a00aa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019b7d8c-aa32-7808-bc4a-1a8bb2f68c2e" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_019b7d8c-aa32-7812-afce-42515f5a00aa" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtTerm_019b7d8c-aa32-7577-8100-25b71fc1b173" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019b7d8c-aa32-7808-bc4a-1a8bb2f68c2e" xlink:to="loc_us-gaap_LongTermDebtTerm_019b7d8c-aa32-7577-8100-25b71fc1b173" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019b7d8c-aa32-7181-8427-a679add4031f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019b7d8c-aa32-7808-bc4a-1a8bb2f68c2e" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019b7d8c-aa32-7181-8427-a679add4031f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019b7d8c-aa32-7ac9-bd7a-b00ff5548cc8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019b7d8c-aa32-7808-bc4a-1a8bb2f68c2e" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019b7d8c-aa32-7ac9-bd7a-b00ff5548cc8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateIncreaseDecrease_019b7d8c-aa32-7849-993b-65f829709524" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentInterestRateIncreaseDecrease"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019b7d8c-aa32-7808-bc4a-1a8bb2f68c2e" xlink:to="loc_us-gaap_DebtInstrumentInterestRateIncreaseDecrease_019b7d8c-aa32-7849-993b-65f829709524" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtWeightedAverageInterestRate_019b7d8c-aa32-7ebf-83ea-229332176b34" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtWeightedAverageInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019b7d8c-aa32-7808-bc4a-1a8bb2f68c2e" xlink:to="loc_us-gaap_DebtWeightedAverageInterestRate_019b7d8c-aa32-7ebf-83ea-229332176b34" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_LineOfCreditFacilityCovenantComplianceDebtUtilizationTriggeringLeverageRatioCompliancePercentage_019b7d8c-aa32-78bd-8789-a239f774e0c1" xlink:href="mgni-20251231.xsd#mgni_LineOfCreditFacilityCovenantComplianceDebtUtilizationTriggeringLeverageRatioCompliancePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019b7d8c-aa32-7808-bc4a-1a8bb2f68c2e" xlink:to="loc_mgni_LineOfCreditFacilityCovenantComplianceDebtUtilizationTriggeringLeverageRatioCompliancePercentage_019b7d8c-aa32-78bd-8789-a239f774e0c1" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_LineOfCreditFacilityCovenantComplianceLeverageRatioMaximum_019b7d8c-aa32-7f03-a691-4fe7b01f1d02" xlink:href="mgni-20251231.xsd#mgni_LineOfCreditFacilityCovenantComplianceLeverageRatioMaximum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019b7d8c-aa32-7808-bc4a-1a8bb2f68c2e" xlink:to="loc_mgni_LineOfCreditFacilityCovenantComplianceLeverageRatioMaximum_019b7d8c-aa32-7f03-a691-4fe7b01f1d02" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCredit_019b7d8c-aa32-7f34-8003-ba7697561d47" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCredit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019b7d8c-aa32-7808-bc4a-1a8bb2f68c2e" xlink:to="loc_us-gaap_LineOfCredit_019b7d8c-aa32-7f34-8003-ba7697561d47" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity_019b7d8c-ed8e-7e02-b411-580447b49ea8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019b7d8c-aa32-7808-bc4a-1a8bb2f68c2e" xlink:to="loc_us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity_019b7d8c-ed8e-7e02-b411-580447b49ea8" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebt_019b7d8c-ed8e-7fbc-a906-650c19b8a9eb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019b7d8c-aa32-7808-bc4a-1a8bb2f68c2e" xlink:to="loc_us-gaap_LongTermDebt_019b7d8c-ed8e-7fbc-a906-650c19b8a9eb" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_DebtInstrumentPeriodicPaymentPercentageOfPrincipal_019b7d8c-ed8e-7f93-a09c-5630e0496b29" xlink:href="mgni-20251231.xsd#mgni_DebtInstrumentPeriodicPaymentPercentageOfPrincipal"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019b7d8c-aa32-7808-bc4a-1a8bb2f68c2e" xlink:to="loc_mgni_DebtInstrumentPeriodicPaymentPercentageOfPrincipal_019b7d8c-ed8e-7f93-a09c-5630e0496b29" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_DebtInstrumentAnnualPrincipalPaymentPercentage_019b7d8c-ed8e-70c5-ab99-5c135f90be04" xlink:href="mgni-20251231.xsd#mgni_DebtInstrumentAnnualPrincipalPaymentPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019b7d8c-aa32-7808-bc4a-1a8bb2f68c2e" xlink:to="loc_mgni_DebtInstrumentAnnualPrincipalPaymentPercentage_019b7d8c-ed8e-70c5-ab99-5c135f90be04" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredFinanceCostsGross_019b7d94-633e-7ea3-bca0-1d23c5595102" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredFinanceCostsGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019b7d8c-aa32-7808-bc4a-1a8bb2f68c2e" xlink:to="loc_us-gaap_DeferredFinanceCostsGross_019b7d94-633e-7ea3-bca0-1d23c5595102" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WriteOffOfDeferredDebtIssuanceCost_019b7d94-633e-73ee-9e0f-b941594e2ce4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WriteOffOfDeferredDebtIssuanceCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019b7d8c-aa32-7808-bc4a-1a8bb2f68c2e" xlink:to="loc_us-gaap_WriteOffOfDeferredDebtIssuanceCost_019b7d94-633e-73ee-9e0f-b941594e2ce4" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainsLossesOnExtinguishmentOfDebt_019b7d94-633e-7cb2-a1a0-75d13968047f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GainsLossesOnExtinguishmentOfDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019b7d8c-aa32-7808-bc4a-1a8bb2f68c2e" xlink:to="loc_us-gaap_GainsLossesOnExtinguishmentOfDebt_019b7d94-633e-7cb2-a1a0-75d13968047f" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFeeAmount_019b7d94-633e-782f-84af-d7f205095d65" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentFeeAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019b7d8c-aa32-7808-bc4a-1a8bb2f68c2e" xlink:to="loc_us-gaap_DebtInstrumentFeeAmount_019b7d94-633e-782f-84af-d7f205095d65" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentUnamortizedDiscount_019b7d94-633e-7587-84e7-7fc2c63cf2dd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentUnamortizedDiscount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019b7d8c-aa32-7808-bc4a-1a8bb2f68c2e" xlink:to="loc_us-gaap_DebtInstrumentUnamortizedDiscount_019b7d94-633e-7587-84e7-7fc2c63cf2dd" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_DebtInstrumentNoncashOrPartNoncashFinancingActivity_019b7d94-633e-7dec-b6f1-c7eb6a1243d4" xlink:href="mgni-20251231.xsd#mgni_DebtInstrumentNoncashOrPartNoncashFinancingActivity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019b7d8c-aa32-7808-bc4a-1a8bb2f68c2e" xlink:to="loc_mgni_DebtInstrumentNoncashOrPartNoncashFinancingActivity_019b7d94-633e-7dec-b6f1-c7eb6a1243d4" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RepaymentsOfSecuredDebt_019b7d94-633e-7b6e-847a-26060abc6e44" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RepaymentsOfSecuredDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LineOfCreditFacilityLineItems_019b7d8c-aa32-7808-bc4a-1a8bb2f68c2e" xlink:to="loc_us-gaap_RepaymentsOfSecuredDebt_019b7d94-633e-7b6e-847a-26060abc6e44" xlink:type="arc" order="19"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/DebtScheduleofTermLoanBFacilityDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#DebtScheduleofTermLoanBFacilityDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/DebtScheduleofTermLoanBFacilityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_98f60178-436f-4eef-99bd-a90e9e095091" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_84c8d8d7-5b68-4494-a76e-e94e9ac8c289" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_98f60178-436f-4eef-99bd-a90e9e095091" xlink:to="loc_us-gaap_DebtInstrumentTable_84c8d8d7-5b68-4494-a76e-e94e9ac8c289" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_48a7c573-61e0-45b5-8886-989922dddc41" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_84c8d8d7-5b68-4494-a76e-e94e9ac8c289" xlink:to="loc_us-gaap_CreditFacilityAxis_48a7c573-61e0-45b5-8886-989922dddc41" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_48a7c573-61e0-45b5-8886-989922dddc41_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_48a7c573-61e0-45b5-8886-989922dddc41" xlink:to="loc_us-gaap_CreditFacilityDomain_48a7c573-61e0-45b5-8886-989922dddc41_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_fdfe9500-1446-4278-8997-57cd77ecd8f7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_48a7c573-61e0-45b5-8886-989922dddc41" xlink:to="loc_us-gaap_CreditFacilityDomain_fdfe9500-1446-4278-8997-57cd77ecd8f7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_7116b22c-cdbd-4bc5-a234-a400b90e68c4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_fdfe9500-1446-4278-8997-57cd77ecd8f7" xlink:to="loc_us-gaap_SecuredDebtMember_7116b22c-cdbd-4bc5-a234-a400b90e68c4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_c3f4fa63-4a1a-41c8-8c3a-bc75b6c137c0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_84c8d8d7-5b68-4494-a76e-e94e9ac8c289" xlink:to="loc_us-gaap_DebtInstrumentAxis_c3f4fa63-4a1a-41c8-8c3a-bc75b6c137c0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_c3f4fa63-4a1a-41c8-8c3a-bc75b6c137c0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_c3f4fa63-4a1a-41c8-8c3a-bc75b6c137c0" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_c3f4fa63-4a1a-41c8-8c3a-bc75b6c137c0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_05911566-de2a-4e87-8008-a15d870a878b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_c3f4fa63-4a1a-41c8-8c3a-bc75b6c137c0" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_05911566-de2a-4e87-8008-a15d870a878b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_CreditAgreement2024Member_019b7d69-d8ac-7f6f-9d03-8be886a080a5" xlink:href="mgni-20251231.xsd#mgni_CreditAgreement2024Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_05911566-de2a-4e87-8008-a15d870a878b" xlink:to="loc_mgni_CreditAgreement2024Member_019b7d69-d8ac-7f6f-9d03-8be886a080a5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_a0581eb2-1f7b-40b0-937b-3534a077b9e4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_84c8d8d7-5b68-4494-a76e-e94e9ac8c289" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_a0581eb2-1f7b-40b0-937b-3534a077b9e4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_a0581eb2-1f7b-40b0-937b-3534a077b9e4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_a0581eb2-1f7b-40b0-937b-3534a077b9e4" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_a0581eb2-1f7b-40b0-937b-3534a077b9e4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_7c014a4c-8938-4557-a928-5676045982a1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_a0581eb2-1f7b-40b0-937b-3534a077b9e4" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_7c014a4c-8938-4557-a928-5676045982a1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_606b8743-919d-465b-a933-9a1aa437b1cc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_7c014a4c-8938-4557-a928-5676045982a1" xlink:to="loc_us-gaap_LineOfCreditMember_606b8743-919d-465b-a933-9a1aa437b1cc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_305828c5-29fd-4806-8add-d43be6c76f26" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_98f60178-436f-4eef-99bd-a90e9e095091" xlink:to="loc_us-gaap_DebtInstrumentCarryingAmount_305828c5-29fd-4806-8add-d43be6c76f26" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentUnamortizedDiscount_b27a224c-6dd5-4a16-a20d-548c24bce5a7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentUnamortizedDiscount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_98f60178-436f-4eef-99bd-a90e9e095091" xlink:to="loc_us-gaap_DebtInstrumentUnamortizedDiscount_b27a224c-6dd5-4a16-a20d-548c24bce5a7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredFinanceCostsNet_68b26339-c6d2-4ef0-b514-ce7ccb6fed62" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredFinanceCostsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_98f60178-436f-4eef-99bd-a90e9e095091" xlink:to="loc_us-gaap_DeferredFinanceCostsNet_68b26339-c6d2-4ef0-b514-ce7ccb6fed62" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtNoncurrent_7546d8b4-d7ab-4dc4-8676-40d4312ea345" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_98f60178-436f-4eef-99bd-a90e9e095091" xlink:to="loc_us-gaap_LongTermDebtNoncurrent_7546d8b4-d7ab-4dc4-8676-40d4312ea345" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/DebtInterestExpenseRelatedtotheTermLoanBFacilityDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#DebtInterestExpenseRelatedtotheTermLoanBFacilityDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/DebtInterestExpenseRelatedtotheTermLoanBFacilityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_588308df-25c6-46fb-8491-84ffbb7cd94c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_ab3f9973-3dfe-4968-99f2-b947ada5152c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_588308df-25c6-46fb-8491-84ffbb7cd94c" xlink:to="loc_us-gaap_DebtInstrumentTable_ab3f9973-3dfe-4968-99f2-b947ada5152c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_7e40aa71-d508-4c15-9f5d-91d93a26793a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_ab3f9973-3dfe-4968-99f2-b947ada5152c" xlink:to="loc_us-gaap_DebtInstrumentAxis_7e40aa71-d508-4c15-9f5d-91d93a26793a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_7e40aa71-d508-4c15-9f5d-91d93a26793a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_7e40aa71-d508-4c15-9f5d-91d93a26793a" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_7e40aa71-d508-4c15-9f5d-91d93a26793a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_c1b847a2-55d1-492f-b57a-92a383d34337" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_7e40aa71-d508-4c15-9f5d-91d93a26793a" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_c1b847a2-55d1-492f-b57a-92a383d34337" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_CreditAgreements2021And2024Member_db94950b-3dbc-4c32-a8fc-b7e5cd80e450" xlink:href="mgni-20251231.xsd#mgni_CreditAgreements2021And2024Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_c1b847a2-55d1-492f-b57a-92a383d34337" xlink:to="loc_mgni_CreditAgreements2021And2024Member_db94950b-3dbc-4c32-a8fc-b7e5cd80e450" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_25698b04-ce39-493b-83a7-e627963e09a6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_ab3f9973-3dfe-4968-99f2-b947ada5152c" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_25698b04-ce39-493b-83a7-e627963e09a6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_25698b04-ce39-493b-83a7-e627963e09a6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_25698b04-ce39-493b-83a7-e627963e09a6" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_25698b04-ce39-493b-83a7-e627963e09a6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_9186aabe-e268-4a34-a5cc-2d0de3d950df" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_25698b04-ce39-493b-83a7-e627963e09a6" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_9186aabe-e268-4a34-a5cc-2d0de3d950df" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_1b5554a9-2300-44aa-b348-a2cdde96461b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_9186aabe-e268-4a34-a5cc-2d0de3d950df" xlink:to="loc_us-gaap_LineOfCreditMember_1b5554a9-2300-44aa-b348-a2cdde96461b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_d34f1665-a968-4908-8672-83460fc3bc2e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_ab3f9973-3dfe-4968-99f2-b947ada5152c" xlink:to="loc_us-gaap_CreditFacilityAxis_d34f1665-a968-4908-8672-83460fc3bc2e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_d34f1665-a968-4908-8672-83460fc3bc2e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_d34f1665-a968-4908-8672-83460fc3bc2e" xlink:to="loc_us-gaap_CreditFacilityDomain_d34f1665-a968-4908-8672-83460fc3bc2e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_52e4dffa-9a7c-4617-b8d0-08a157eab496" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_d34f1665-a968-4908-8672-83460fc3bc2e" xlink:to="loc_us-gaap_CreditFacilityDomain_52e4dffa-9a7c-4617-b8d0-08a157eab496" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_780a245d-1d06-4d65-aad2-1243a5f76b23" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_52e4dffa-9a7c-4617-b8d0-08a157eab496" xlink:to="loc_us-gaap_SecuredDebtMember_780a245d-1d06-4d65-aad2-1243a5f76b23" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseDebt_76431936-efbc-4e03-84a7-645c100f5521" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestExpenseDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_588308df-25c6-46fb-8491-84ffbb7cd94c" xlink:to="loc_us-gaap_InterestExpenseDebt_76431936-efbc-4e03-84a7-645c100f5521" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfDebtDiscountPremium_7e6ffa04-a66f-4fd5-ae23-c15fec022614" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AmortizationOfDebtDiscountPremium"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_588308df-25c6-46fb-8491-84ffbb7cd94c" xlink:to="loc_us-gaap_AmortizationOfDebtDiscountPremium_7e6ffa04-a66f-4fd5-ae23-c15fec022614" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfFinancingCosts_e0a0433d-bafb-4a80-bfc1-2e64bad7a46c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AmortizationOfFinancingCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_588308df-25c6-46fb-8491-84ffbb7cd94c" xlink:to="loc_us-gaap_AmortizationOfFinancingCosts_e0a0433d-bafb-4a80-bfc1-2e64bad7a46c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpense_4bd2a40d-9ff1-4c28-b20e-b8d62e31e570" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_588308df-25c6-46fb-8491-84ffbb7cd94c" xlink:to="loc_us-gaap_InterestExpense_4bd2a40d-9ff1-4c28-b20e-b8d62e31e570" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateDuringPeriod_ed5380ce-2477-4850-8853-d0774ca3c66a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentInterestRateDuringPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_588308df-25c6-46fb-8491-84ffbb7cd94c" xlink:to="loc_us-gaap_DebtInstrumentInterestRateDuringPeriod_ed5380ce-2477-4850-8853-d0774ca3c66a" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/DebtAmortizationExpensefortheTermLoanBFacilityDebtDiscountandIssuanceCostsDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#DebtAmortizationExpensefortheTermLoanBFacilityDebtDiscountandIssuanceCostsDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/DebtAmortizationExpensefortheTermLoanBFacilityDebtDiscountandIssuanceCostsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_1e7679e9-a022-46d0-a3a3-2bc2985a48f2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_85c1baf0-80b9-4968-bc6e-7694d806851a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_1e7679e9-a022-46d0-a3a3-2bc2985a48f2" xlink:to="loc_us-gaap_DebtInstrumentTable_85c1baf0-80b9-4968-bc6e-7694d806851a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_1fd2c3fe-0a10-4cc9-83c0-b2a5e5818d06" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_85c1baf0-80b9-4968-bc6e-7694d806851a" xlink:to="loc_us-gaap_DebtInstrumentAxis_1fd2c3fe-0a10-4cc9-83c0-b2a5e5818d06" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_1fd2c3fe-0a10-4cc9-83c0-b2a5e5818d06_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_1fd2c3fe-0a10-4cc9-83c0-b2a5e5818d06" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_1fd2c3fe-0a10-4cc9-83c0-b2a5e5818d06_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_7d109e7d-2843-4624-83a7-0cd87a30a67a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_1fd2c3fe-0a10-4cc9-83c0-b2a5e5818d06" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_7d109e7d-2843-4624-83a7-0cd87a30a67a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_CreditAgreement2024Member_45ea7993-7c1d-44c1-b331-dd1d80d8e95a" xlink:href="mgni-20251231.xsd#mgni_CreditAgreement2024Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_7d109e7d-2843-4624-83a7-0cd87a30a67a" xlink:to="loc_mgni_CreditAgreement2024Member_45ea7993-7c1d-44c1-b331-dd1d80d8e95a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_4e5d05ea-88e9-4dee-a2ed-9e596095234d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_85c1baf0-80b9-4968-bc6e-7694d806851a" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_4e5d05ea-88e9-4dee-a2ed-9e596095234d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_4e5d05ea-88e9-4dee-a2ed-9e596095234d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_4e5d05ea-88e9-4dee-a2ed-9e596095234d" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_4e5d05ea-88e9-4dee-a2ed-9e596095234d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_b6daeb09-9ba4-42b9-8c47-93afb89d6918" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_4e5d05ea-88e9-4dee-a2ed-9e596095234d" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_b6daeb09-9ba4-42b9-8c47-93afb89d6918" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_3ad09833-4c94-4a8c-8dbf-0ec294c3c19a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_b6daeb09-9ba4-42b9-8c47-93afb89d6918" xlink:to="loc_us-gaap_LineOfCreditMember_3ad09833-4c94-4a8c-8dbf-0ec294c3c19a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_8f4b2371-f776-40d9-9fd2-6951887b0069" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_85c1baf0-80b9-4968-bc6e-7694d806851a" xlink:to="loc_us-gaap_CreditFacilityAxis_8f4b2371-f776-40d9-9fd2-6951887b0069" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_8f4b2371-f776-40d9-9fd2-6951887b0069_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_8f4b2371-f776-40d9-9fd2-6951887b0069" xlink:to="loc_us-gaap_CreditFacilityDomain_8f4b2371-f776-40d9-9fd2-6951887b0069_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_45e735b6-895b-4f48-919f-0b6511b94737" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_8f4b2371-f776-40d9-9fd2-6951887b0069" xlink:to="loc_us-gaap_CreditFacilityDomain_45e735b6-895b-4f48-919f-0b6511b94737" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_eee0bd95-eb26-478e-8950-74587a93faac" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_45e735b6-895b-4f48-919f-0b6511b94737" xlink:to="loc_us-gaap_SecuredDebtMember_eee0bd95-eb26-478e-8950-74587a93faac" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_AmortizationOfDebtDiscountsAbstract_ce46d1a3-a1c1-4dbc-98d5-9ceb7b2ec709" xlink:href="mgni-20251231.xsd#mgni_AmortizationOfDebtDiscountsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_1e7679e9-a022-46d0-a3a3-2bc2985a48f2" xlink:to="loc_mgni_AmortizationOfDebtDiscountsAbstract_ce46d1a3-a1c1-4dbc-98d5-9ceb7b2ec709" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_AmortizationOfDebtDiscountPremiumYearOne_e5240c56-1f12-43fa-a8fd-1b2883a9a5df" xlink:href="mgni-20251231.xsd#mgni_AmortizationOfDebtDiscountPremiumYearOne"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_AmortizationOfDebtDiscountsAbstract_ce46d1a3-a1c1-4dbc-98d5-9ceb7b2ec709" xlink:to="loc_mgni_AmortizationOfDebtDiscountPremiumYearOne_e5240c56-1f12-43fa-a8fd-1b2883a9a5df" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_AmortizationOfDebtDiscountPremiumYearTwo_366c7756-38dd-4561-af04-8bb195c15ad7" xlink:href="mgni-20251231.xsd#mgni_AmortizationOfDebtDiscountPremiumYearTwo"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_AmortizationOfDebtDiscountsAbstract_ce46d1a3-a1c1-4dbc-98d5-9ceb7b2ec709" xlink:to="loc_mgni_AmortizationOfDebtDiscountPremiumYearTwo_366c7756-38dd-4561-af04-8bb195c15ad7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_AmortizationOfDebtDiscountPremiumYearThree_73f65bff-1340-4068-930a-19f6c745e7dc" xlink:href="mgni-20251231.xsd#mgni_AmortizationOfDebtDiscountPremiumYearThree"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_AmortizationOfDebtDiscountsAbstract_ce46d1a3-a1c1-4dbc-98d5-9ceb7b2ec709" xlink:to="loc_mgni_AmortizationOfDebtDiscountPremiumYearThree_73f65bff-1340-4068-930a-19f6c745e7dc" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_AmortizationOfDebtDiscountPremiumYearFour_7b9fdf29-1df4-4924-b4b8-4b085617cc9b" xlink:href="mgni-20251231.xsd#mgni_AmortizationOfDebtDiscountPremiumYearFour"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_AmortizationOfDebtDiscountsAbstract_ce46d1a3-a1c1-4dbc-98d5-9ceb7b2ec709" xlink:to="loc_mgni_AmortizationOfDebtDiscountPremiumYearFour_7b9fdf29-1df4-4924-b4b8-4b085617cc9b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_AmortizationOfDebtDiscountPremiumYearFive_dcda2dfe-24bc-4274-990a-5b7c084dcf34" xlink:href="mgni-20251231.xsd#mgni_AmortizationOfDebtDiscountPremiumYearFive"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_AmortizationOfDebtDiscountsAbstract_ce46d1a3-a1c1-4dbc-98d5-9ceb7b2ec709" xlink:to="loc_mgni_AmortizationOfDebtDiscountPremiumYearFive_dcda2dfe-24bc-4274-990a-5b7c084dcf34" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_AmortizationOfDebtDiscountPremiumAfterYearFive_4143e55b-2562-44cc-be0e-26079524a856" xlink:href="mgni-20251231.xsd#mgni_AmortizationOfDebtDiscountPremiumAfterYearFive"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_AmortizationOfDebtDiscountsAbstract_ce46d1a3-a1c1-4dbc-98d5-9ceb7b2ec709" xlink:to="loc_mgni_AmortizationOfDebtDiscountPremiumAfterYearFive_4143e55b-2562-44cc-be0e-26079524a856" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentUnamortizedDiscount_e1431aaf-4936-4c3f-89f4-9f20ea6a88d1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentUnamortizedDiscount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_AmortizationOfDebtDiscountsAbstract_ce46d1a3-a1c1-4dbc-98d5-9ceb7b2ec709" xlink:to="loc_us-gaap_DebtInstrumentUnamortizedDiscount_e1431aaf-4936-4c3f-89f4-9f20ea6a88d1" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_AmortizationOfDebtIssuanceCostsAbstract_d8974766-9f69-4e5d-ab08-700e917a6307" xlink:href="mgni-20251231.xsd#mgni_AmortizationOfDebtIssuanceCostsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_1e7679e9-a022-46d0-a3a3-2bc2985a48f2" xlink:to="loc_mgni_AmortizationOfDebtIssuanceCostsAbstract_d8974766-9f69-4e5d-ab08-700e917a6307" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_AmortizationOfDebtIssuanceCostsYearOne_019b7d5f-7e2f-798b-a8a3-c91de4e89ec6" xlink:href="mgni-20251231.xsd#mgni_AmortizationOfDebtIssuanceCostsYearOne"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_AmortizationOfDebtIssuanceCostsAbstract_d8974766-9f69-4e5d-ab08-700e917a6307" xlink:to="loc_mgni_AmortizationOfDebtIssuanceCostsYearOne_019b7d5f-7e2f-798b-a8a3-c91de4e89ec6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_AmortizationOfDebtIssuanceCostsYearTwo_019b7d5f-7e2f-709d-9c48-fc591dcdfe64" xlink:href="mgni-20251231.xsd#mgni_AmortizationOfDebtIssuanceCostsYearTwo"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_AmortizationOfDebtIssuanceCostsAbstract_d8974766-9f69-4e5d-ab08-700e917a6307" xlink:to="loc_mgni_AmortizationOfDebtIssuanceCostsYearTwo_019b7d5f-7e2f-709d-9c48-fc591dcdfe64" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_AmortizationOfDebtIssuanceCostsYearThree_d405d96e-4dd9-4425-906f-e06a4cf7224f" xlink:href="mgni-20251231.xsd#mgni_AmortizationOfDebtIssuanceCostsYearThree"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_AmortizationOfDebtIssuanceCostsAbstract_d8974766-9f69-4e5d-ab08-700e917a6307" xlink:to="loc_mgni_AmortizationOfDebtIssuanceCostsYearThree_d405d96e-4dd9-4425-906f-e06a4cf7224f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_AmortizationOfDebtIssuanceCostsYearFour_430f3177-d818-4a50-84d3-d8472e66eadc" xlink:href="mgni-20251231.xsd#mgni_AmortizationOfDebtIssuanceCostsYearFour"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_AmortizationOfDebtIssuanceCostsAbstract_d8974766-9f69-4e5d-ab08-700e917a6307" xlink:to="loc_mgni_AmortizationOfDebtIssuanceCostsYearFour_430f3177-d818-4a50-84d3-d8472e66eadc" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_AmortizationOfDebtIssuanceCostsYearFive_c659d1ee-ec3c-4f11-bf2d-61189cbeb397" xlink:href="mgni-20251231.xsd#mgni_AmortizationOfDebtIssuanceCostsYearFive"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_AmortizationOfDebtIssuanceCostsAbstract_d8974766-9f69-4e5d-ab08-700e917a6307" xlink:to="loc_mgni_AmortizationOfDebtIssuanceCostsYearFive_c659d1ee-ec3c-4f11-bf2d-61189cbeb397" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_AmortizationOfDebtIssuanceCostsAfterYearFive_65b63c98-9c7e-4ba3-aa39-ab5c5f05f9f5" xlink:href="mgni-20251231.xsd#mgni_AmortizationOfDebtIssuanceCostsAfterYearFive"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_AmortizationOfDebtIssuanceCostsAbstract_d8974766-9f69-4e5d-ab08-700e917a6307" xlink:to="loc_mgni_AmortizationOfDebtIssuanceCostsAfterYearFive_65b63c98-9c7e-4ba3-aa39-ab5c5f05f9f5" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredFinanceCostsNet_dba4d252-a450-4393-b7e3-b69da4ce3a0f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredFinanceCostsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mgni_AmortizationOfDebtIssuanceCostsAbstract_d8974766-9f69-4e5d-ab08-700e917a6307" xlink:to="loc_us-gaap_DeferredFinanceCostsNet_dba4d252-a450-4393-b7e3-b69da4ce3a0f" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/CommitmentsandContingenciesNarrativeDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#CommitmentsandContingenciesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/CommitmentsandContingenciesNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCommitmentsLineItems_664899d9-29dc-418e-8075-3b923093a7f4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherCommitmentsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCommitmentsTable_2d0638f4-8716-4db9-b365-b3acdf2778fb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherCommitmentsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_664899d9-29dc-418e-8075-3b923093a7f4" xlink:to="loc_us-gaap_OtherCommitmentsTable_2d0638f4-8716-4db9-b365-b3acdf2778fb" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesByNatureOfContingencyAxis_df749ffe-2a56-40e0-bcbb-78b783f50d9b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LossContingenciesByNatureOfContingencyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OtherCommitmentsTable_2d0638f4-8716-4db9-b365-b3acdf2778fb" xlink:to="loc_us-gaap_LossContingenciesByNatureOfContingencyAxis_df749ffe-2a56-40e0-bcbb-78b783f50d9b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingencyNatureDomain_df749ffe-2a56-40e0-bcbb-78b783f50d9b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LossContingencyNatureDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LossContingenciesByNatureOfContingencyAxis_df749ffe-2a56-40e0-bcbb-78b783f50d9b" xlink:to="loc_us-gaap_LossContingencyNatureDomain_df749ffe-2a56-40e0-bcbb-78b783f50d9b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingencyNatureDomain_7f3eb48a-b7e1-416a-815c-fceb9d798227" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LossContingencyNatureDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LossContingenciesByNatureOfContingencyAxis_df749ffe-2a56-40e0-bcbb-78b783f50d9b" xlink:to="loc_us-gaap_LossContingencyNatureDomain_7f3eb48a-b7e1-416a-815c-fceb9d798227" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialStandbyLetterOfCreditMember_c55a49b0-904c-4477-8f4d-828413c926eb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialStandbyLetterOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingencyNatureDomain_7f3eb48a-b7e1-416a-815c-fceb9d798227" xlink:to="loc_us-gaap_FinancialStandbyLetterOfCreditMember_c55a49b0-904c-4477-8f4d-828413c926eb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LettersOfCreditOutstandingAmount_c882538e-80a4-47a1-ab22-29f3580d5710" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LettersOfCreditOutstandingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsLineItems_664899d9-29dc-418e-8075-3b923093a7f4" xlink:to="loc_us-gaap_LettersOfCreditOutstandingAmount_c882538e-80a4-47a1-ab22-29f3580d5710" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/AccumulatedOtherComprehensiveLossScheduleofComponentsofAccumulatedOtherComprehensiveLossDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#AccumulatedOtherComprehensiveLossScheduleofComponentsofAccumulatedOtherComprehensiveLossDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/AccumulatedOtherComprehensiveLossScheduleofComponentsofAccumulatedOtherComprehensiveLossDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_89dca9f5-db44-4dc1-844f-f8d4e77dd939" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_28e91949-ac28-4c5c-9b14-1308cc8e6ec2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_89dca9f5-db44-4dc1-844f-f8d4e77dd939" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_28e91949-ac28-4c5c-9b14-1308cc8e6ec2" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_c1441afe-0098-47d1-b788-f20a1c10bb28" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_28e91949-ac28-4c5c-9b14-1308cc8e6ec2" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_c1441afe-0098-47d1-b788-f20a1c10bb28" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_c1441afe-0098-47d1-b788-f20a1c10bb28_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_c1441afe-0098-47d1-b788-f20a1c10bb28" xlink:to="loc_us-gaap_EquityComponentDomain_c1441afe-0098-47d1-b788-f20a1c10bb28_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_ccd4573d-2249-4693-ac15-dff8a6fcccea" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_c1441afe-0098-47d1-b788-f20a1c10bb28" xlink:to="loc_us-gaap_EquityComponentDomain_ccd4573d-2249-4693-ac15-dff8a6fcccea" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_845f3313-9778-4874-bcf6-1d5aa4abe608" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_ccd4573d-2249-4693-ac15-dff8a6fcccea" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_845f3313-9778-4874-bcf6-1d5aa4abe608" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AOCIAttributableToParentNetOfTaxRollForward_194689bd-ab2e-40dd-99b5-26298d133466" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AOCIAttributableToParentNetOfTaxRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_89dca9f5-db44-4dc1-844f-f8d4e77dd939" xlink:to="loc_us-gaap_AOCIAttributableToParentNetOfTaxRollForward_194689bd-ab2e-40dd-99b5-26298d133466" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_a54e474f-158e-4f74-9f2d-c1a367af4e1a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AOCIAttributableToParentNetOfTaxRollForward_194689bd-ab2e-40dd-99b5-26298d133466" xlink:to="loc_us-gaap_StockholdersEquity_a54e474f-158e-4f74-9f2d-c1a367af4e1a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_d2646cb4-4ef6-4d67-bb9c-8a32f20a1270" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AOCIAttributableToParentNetOfTaxRollForward_194689bd-ab2e-40dd-99b5-26298d133466" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_d2646cb4-4ef6-4d67-bb9c-8a32f20a1270" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_d19cd5db-65d0-41e1-acab-d1cef1e7c579" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/StockBasedCompensationNarrativeDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#StockBasedCompensationNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/StockBasedCompensationNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_eb828612-10c0-40d1-ada0-992c3a53619a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_15f3a4c5-bbc5-464d-980b-93a3266d4984" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_eb828612-10c0-40d1-ada0-992c3a53619a" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_15f3a4c5-bbc5-464d-980b-93a3266d4984" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_c513c426-49c8-46f0-9f66-b99aea786bd8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_15f3a4c5-bbc5-464d-980b-93a3266d4984" xlink:to="loc_us-gaap_AwardTypeAxis_c513c426-49c8-46f0-9f66-b99aea786bd8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_c513c426-49c8-46f0-9f66-b99aea786bd8_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_c513c426-49c8-46f0-9f66-b99aea786bd8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_c513c426-49c8-46f0-9f66-b99aea786bd8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_cd56847d-d4d1-4614-bfbb-5d5dbdc81eb9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_c513c426-49c8-46f0-9f66-b99aea786bd8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_cd56847d-d4d1-4614-bfbb-5d5dbdc81eb9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_5ab033c5-5b95-4d2e-af3e-786f2ac1d5da" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_cd56847d-d4d1-4614-bfbb-5d5dbdc81eb9" xlink:to="loc_us-gaap_EmployeeStockOptionMember_5ab033c5-5b95-4d2e-af3e-786f2ac1d5da" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_RestrictedawardsMember_cbb2401c-f157-4f45-b5f6-6029f9ecd75f" xlink:href="mgni-20251231.xsd#mgni_RestrictedawardsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_cd56847d-d4d1-4614-bfbb-5d5dbdc81eb9" xlink:to="loc_mgni_RestrictedawardsMember_cbb2401c-f157-4f45-b5f6-6029f9ecd75f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingAxis_2e98e167-6dd3-4097-93fd-d41fdbaa483f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VestingAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_15f3a4c5-bbc5-464d-980b-93a3266d4984" xlink:to="loc_us-gaap_VestingAxis_2e98e167-6dd3-4097-93fd-d41fdbaa483f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_2e98e167-6dd3-4097-93fd-d41fdbaa483f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VestingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VestingAxis_2e98e167-6dd3-4097-93fd-d41fdbaa483f" xlink:to="loc_us-gaap_VestingDomain_2e98e167-6dd3-4097-93fd-d41fdbaa483f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_f5d88ae6-c28f-4afc-bf22-6878239f5fc0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VestingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VestingAxis_2e98e167-6dd3-4097-93fd-d41fdbaa483f" xlink:to="loc_us-gaap_VestingDomain_f5d88ae6-c28f-4afc-bf22-6878239f5fc0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_dbab11a3-88ce-4927-a368-86a5a2679366" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationAwardTrancheOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_f5d88ae6-c28f-4afc-bf22-6878239f5fc0" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_dbab11a3-88ce-4927-a368-86a5a2679366" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_019c91cd-abc1-7574-a2d5-3dc0a965ceed" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_15f3a4c5-bbc5-464d-980b-93a3266d4984" xlink:to="loc_us-gaap_PlanNameAxis_019c91cd-abc1-7574-a2d5-3dc0a965ceed" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019c91cd-abc1-7574-a2d5-3dc0a965ceed_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_019c91cd-abc1-7574-a2d5-3dc0a965ceed" xlink:to="loc_us-gaap_PlanNameDomain_019c91cd-abc1-7574-a2d5-3dc0a965ceed_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_019c91cd-abc1-7bd9-8ff4-9919790966a3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_019c91cd-abc1-7574-a2d5-3dc0a965ceed" xlink:to="loc_us-gaap_PlanNameDomain_019c91cd-abc1-7bd9-8ff4-9919790966a3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_AmendedAndRestated2014EquityIncentivePlanMember_019c91cd-abc1-7112-8fb1-d121dfbda4bd" xlink:href="mgni-20251231.xsd#mgni_AmendedAndRestated2014EquityIncentivePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_019c91cd-abc1-7bd9-8ff4-9919790966a3" xlink:to="loc_mgni_AmendedAndRestated2014EquityIncentivePlanMember_019c91cd-abc1-7112-8fb1-d121dfbda4bd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_bf94cd60-f056-4b3b-94e6-389d67dba47f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_eb828612-10c0-40d1-ada0-992c3a53619a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_bf94cd60-f056-4b3b-94e6-389d67dba47f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_ef1c7b49-03f5-40bc-8c1d-1038ae19b0af" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_eb828612-10c0-40d1-ada0-992c3a53619a" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_ef1c7b49-03f5-40bc-8c1d-1038ae19b0af" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_cc666012-8965-4bbd-bc7b-dce93a19735a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_eb828612-10c0-40d1-ada0-992c3a53619a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_cc666012-8965-4bbd-bc7b-dce93a19735a" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/StockBasedCompensationStockOptionsNarrativeDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#StockBasedCompensationStockOptionsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/StockBasedCompensationStockOptionsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_849fd381-173b-4360-8629-bed93d3e1d44" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_f896a11d-374f-4621-843a-cc7b2e27ea3e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_849fd381-173b-4360-8629-bed93d3e1d44" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_f896a11d-374f-4621-843a-cc7b2e27ea3e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_6aa29cbd-7790-42ad-8556-8fd4a5d6cfe2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_f896a11d-374f-4621-843a-cc7b2e27ea3e" xlink:to="loc_us-gaap_AwardTypeAxis_6aa29cbd-7790-42ad-8556-8fd4a5d6cfe2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_6aa29cbd-7790-42ad-8556-8fd4a5d6cfe2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_6aa29cbd-7790-42ad-8556-8fd4a5d6cfe2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_6aa29cbd-7790-42ad-8556-8fd4a5d6cfe2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_ee720428-461c-4f35-a416-ab478b6dea85" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_6aa29cbd-7790-42ad-8556-8fd4a5d6cfe2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_ee720428-461c-4f35-a416-ab478b6dea85" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_4b6033f9-410b-4523-a9ae-47bf176d2cee" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_ee720428-461c-4f35-a416-ab478b6dea85" xlink:to="loc_us-gaap_EmployeeStockOptionMember_4b6033f9-410b-4523-a9ae-47bf176d2cee" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_16f14ead-36a2-47ff-9686-44c593f1b431" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_849fd381-173b-4360-8629-bed93d3e1d44" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_16f14ead-36a2-47ff-9686-44c593f1b431" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_66250f06-a1d5-4006-a707-a91009077d1b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_849fd381-173b-4360-8629-bed93d3e1d44" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_66250f06-a1d5-4006-a707-a91009077d1b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_2c7c5582-21d2-4583-a4bb-7c26bc34ce3e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_849fd381-173b-4360-8629-bed93d3e1d44" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_2c7c5582-21d2-4583-a4bb-7c26bc34ce3e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_fd26880c-9b74-4d0e-b1dc-4418d317ba34" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_849fd381-173b-4360-8629-bed93d3e1d44" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_fd26880c-9b74-4d0e-b1dc-4418d317ba34" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/StockBasedCompensationScheduleofWeightedAverageInputAssumptionsDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#StockBasedCompensationScheduleofWeightedAverageInputAssumptionsDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/StockBasedCompensationScheduleofWeightedAverageInputAssumptionsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_370ca7fb-79b0-41cb-9972-9df44f44c8ec" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_9093e086-aa2b-46d5-a73a-ffab28c5393d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_370ca7fb-79b0-41cb-9972-9df44f44c8ec" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_9093e086-aa2b-46d5-a73a-ffab28c5393d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_9221e1b8-2ca2-4e61-9715-ee1eededa890" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_9093e086-aa2b-46d5-a73a-ffab28c5393d" xlink:to="loc_us-gaap_AwardTypeAxis_9221e1b8-2ca2-4e61-9715-ee1eededa890" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_9221e1b8-2ca2-4e61-9715-ee1eededa890_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_9221e1b8-2ca2-4e61-9715-ee1eededa890" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_9221e1b8-2ca2-4e61-9715-ee1eededa890_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_bdc4aacc-5d16-4967-acdd-2841ff8ec91e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_9221e1b8-2ca2-4e61-9715-ee1eededa890" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_bdc4aacc-5d16-4967-acdd-2841ff8ec91e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_957bd9a6-51d0-4e1b-97ba-d1eb7b2f5e4a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_bdc4aacc-5d16-4967-acdd-2841ff8ec91e" xlink:to="loc_us-gaap_EmployeeStockOptionMember_957bd9a6-51d0-4e1b-97ba-d1eb7b2f5e4a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_46c31620-4e00-4262-9e4c-82fe83df30a7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_370ca7fb-79b0-41cb-9972-9df44f44c8ec" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_46c31620-4e00-4262-9e4c-82fe83df30a7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_99f6c3f5-125c-4a3a-871b-d5ee921dcc01" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_370ca7fb-79b0-41cb-9972-9df44f44c8ec" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_99f6c3f5-125c-4a3a-871b-d5ee921dcc01" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_c8f00a40-f54d-4b31-b188-ec51ff977acb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_370ca7fb-79b0-41cb-9972-9df44f44c8ec" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_c8f00a40-f54d-4b31-b188-ec51ff977acb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_6c3fbe50-2468-44fc-b5d1-77e4b22a0ea0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_370ca7fb-79b0-41cb-9972-9df44f44c8ec" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_6c3fbe50-2468-44fc-b5d1-77e4b22a0ea0" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/StockBasedCompensationScheduleofRestrictedStockUnitsActivityDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#StockBasedCompensationScheduleofRestrictedStockUnitsActivityDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/StockBasedCompensationScheduleofRestrictedStockUnitsActivityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6bbc9cb7-e220-42d0-aa74-07336a895fc8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_7f08d5f1-346c-4436-9477-c61a3c54fa8b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6bbc9cb7-e220-42d0-aa74-07336a895fc8" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_7f08d5f1-346c-4436-9477-c61a3c54fa8b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_12ec24d9-0df4-46c6-8598-565dc9133999" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_7f08d5f1-346c-4436-9477-c61a3c54fa8b" xlink:to="loc_us-gaap_AwardTypeAxis_12ec24d9-0df4-46c6-8598-565dc9133999" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_12ec24d9-0df4-46c6-8598-565dc9133999_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_12ec24d9-0df4-46c6-8598-565dc9133999" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_12ec24d9-0df4-46c6-8598-565dc9133999_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_3ebe728f-6243-423c-9871-84a2bf5c6ba8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_12ec24d9-0df4-46c6-8598-565dc9133999" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_3ebe728f-6243-423c-9871-84a2bf5c6ba8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_d79a8b20-b5fb-4555-b048-11b3570dff32" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_3ebe728f-6243-423c-9871-84a2bf5c6ba8" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_d79a8b20-b5fb-4555-b048-11b3570dff32" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_2f182b09-3e3e-43a0-81c6-7d6840b8580b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6bbc9cb7-e220-42d0-aa74-07336a895fc8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_2f182b09-3e3e-43a0-81c6-7d6840b8580b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsUnvestedAndVestedButDeferredNumber_73fef12d-cb9a-4243-8ca2-084aa2cf6d11" xlink:href="mgni-20251231.xsd#mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsUnvestedAndVestedButDeferredNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_2f182b09-3e3e-43a0-81c6-7d6840b8580b" xlink:to="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsUnvestedAndVestedButDeferredNumber_73fef12d-cb9a-4243-8ca2-084aa2cf6d11" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_f4e3e2a6-9152-4fec-a75b-f6716073b42f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_2f182b09-3e3e-43a0-81c6-7d6840b8580b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_f4e3e2a6-9152-4fec-a75b-f6716073b42f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_17f9e367-886b-44a8-abac-6bba36cb8212" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_2f182b09-3e3e-43a0-81c6-7d6840b8580b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_17f9e367-886b-44a8-abac-6bba36cb8212" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_aef343bd-909e-4346-9dca-ce32a1beba79" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_2f182b09-3e3e-43a0-81c6-7d6840b8580b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_aef343bd-909e-4346-9dca-ce32a1beba79" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsUnvestedAndVestedButDeferredNumber_604ee115-0971-40a5-a439-671086ec474f" xlink:href="mgni-20251231.xsd#mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsUnvestedAndVestedButDeferredNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_4f6c6069-f447-4b27-84a5-2e2973ad6f26" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6bbc9cb7-e220-42d0-aa74-07336a895fc8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_4f6c6069-f447-4b27-84a5-2e2973ad6f26" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedAndVestedButDeferredWeightedAverageGrantDateFairValue_024ef3f0-4a9c-44d2-86cb-c562072602eb" xlink:href="mgni-20251231.xsd#mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedAndVestedButDeferredWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_4f6c6069-f447-4b27-84a5-2e2973ad6f26" xlink:to="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedAndVestedButDeferredWeightedAverageGrantDateFairValue_024ef3f0-4a9c-44d2-86cb-c562072602eb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_3fcaed95-b8bd-4f6c-973d-2a5dc8818af3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_4f6c6069-f447-4b27-84a5-2e2973ad6f26" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_3fcaed95-b8bd-4f6c-973d-2a5dc8818af3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_a0638b32-abb7-4d50-8c14-3a1e29d45125" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_4f6c6069-f447-4b27-84a5-2e2973ad6f26" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_a0638b32-abb7-4d50-8c14-3a1e29d45125" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_a4e94f79-2944-40fe-8dfd-f6958de5f0a7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_4f6c6069-f447-4b27-84a5-2e2973ad6f26" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_a4e94f79-2944-40fe-8dfd-f6958de5f0a7" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedAndVestedButDeferredWeightedAverageGrantDateFairValue_2b302431-f43c-4bfc-9035-2f0a80d3aa51" xlink:href="mgni-20251231.xsd#mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedAndVestedButDeferredWeightedAverageGrantDateFairValue"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/StockBasedCompensationRestrictedStockUnitsNarrativeDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#StockBasedCompensationRestrictedStockUnitsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/StockBasedCompensationRestrictedStockUnitsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_e5132be2-2dbe-4329-823e-1aefe4dfe446" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_bd8112bf-080c-449e-8ae7-db93aff56d4e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_e5132be2-2dbe-4329-823e-1aefe4dfe446" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_bd8112bf-080c-449e-8ae7-db93aff56d4e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_cc49778b-2d63-4da4-9b71-42f202e5d246" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_bd8112bf-080c-449e-8ae7-db93aff56d4e" xlink:to="loc_us-gaap_AwardTypeAxis_cc49778b-2d63-4da4-9b71-42f202e5d246" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_cc49778b-2d63-4da4-9b71-42f202e5d246_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_cc49778b-2d63-4da4-9b71-42f202e5d246" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_cc49778b-2d63-4da4-9b71-42f202e5d246_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_28f5d4b1-ab7f-4c6f-b3bd-1cc3f9736bac" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_cc49778b-2d63-4da4-9b71-42f202e5d246" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_28f5d4b1-ab7f-4c6f-b3bd-1cc3f9736bac" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_7c349e7c-85b9-473a-be95-01993b1d2f66" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_28f5d4b1-ab7f-4c6f-b3bd-1cc3f9736bac" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_7c349e7c-85b9-473a-be95-01993b1d2f66" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_860bc973-04da-4270-93c3-30d7c73de771" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_e5132be2-2dbe-4329-823e-1aefe4dfe446" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_860bc973-04da-4270-93c3-30d7c73de771" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue_7e547fca-a479-49b6-94aa-948bede654f9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_e5132be2-2dbe-4329-823e-1aefe4dfe446" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue_7e547fca-a479-49b6-94aa-948bede654f9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueNonvested_1addda8f-16fb-4b56-8e0c-7389e94bfa7a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueNonvested"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_e5132be2-2dbe-4329-823e-1aefe4dfe446" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueNonvested_1addda8f-16fb-4b56-8e0c-7389e94bfa7a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_d0acdc9e-9a0f-4406-8495-c2610a287c8e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_e5132be2-2dbe-4329-823e-1aefe4dfe446" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_d0acdc9e-9a0f-4406-8495-c2610a287c8e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_8d5b70b4-0d98-4c2b-a2e7-651264c9e0c4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_e5132be2-2dbe-4329-823e-1aefe4dfe446" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_8d5b70b4-0d98-4c2b-a2e7-651264c9e0c4" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/StockBasedCompensationPerformanceStockUnitsNarrativeDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#StockBasedCompensationPerformanceStockUnitsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/StockBasedCompensationPerformanceStockUnitsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_c789e798-2e35-41e5-bdc1-59bf7ef42465" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_41efb38f-7f74-42c3-aeb1-1699aa8d5e1e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_c789e798-2e35-41e5-bdc1-59bf7ef42465" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_41efb38f-7f74-42c3-aeb1-1699aa8d5e1e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_b032fd5b-2d07-4752-ac97-77d5495f7ec3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_41efb38f-7f74-42c3-aeb1-1699aa8d5e1e" xlink:to="loc_us-gaap_AwardTypeAxis_b032fd5b-2d07-4752-ac97-77d5495f7ec3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_b032fd5b-2d07-4752-ac97-77d5495f7ec3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_b032fd5b-2d07-4752-ac97-77d5495f7ec3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_b032fd5b-2d07-4752-ac97-77d5495f7ec3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_9a8a4e40-c2a5-4670-9fcd-f0fe4187cefc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_b032fd5b-2d07-4752-ac97-77d5495f7ec3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_9a8a4e40-c2a5-4670-9fcd-f0fe4187cefc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_4a02b6ae-8ecb-4070-9a86-7fcca3d00993" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_9a8a4e40-c2a5-4670-9fcd-f0fe4187cefc" xlink:to="loc_us-gaap_PerformanceSharesMember_4a02b6ae-8ecb-4070-9a86-7fcca3d00993" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_b0d19629-1951-4cf9-b874-ccb1e0c6615d" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_41efb38f-7f74-42c3-aeb1-1699aa8d5e1e" xlink:to="loc_srt_RangeAxis_b0d19629-1951-4cf9-b874-ccb1e0c6615d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_b0d19629-1951-4cf9-b874-ccb1e0c6615d_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_b0d19629-1951-4cf9-b874-ccb1e0c6615d" xlink:to="loc_srt_RangeMember_b0d19629-1951-4cf9-b874-ccb1e0c6615d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_deb0a21b-2f95-4383-86b8-dc842c9292d2" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_b0d19629-1951-4cf9-b874-ccb1e0c6615d" xlink:to="loc_srt_RangeMember_deb0a21b-2f95-4383-86b8-dc842c9292d2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_20646a82-3b00-4112-829b-5a190f37a00f" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_deb0a21b-2f95-4383-86b8-dc842c9292d2" xlink:to="loc_srt_MinimumMember_20646a82-3b00-4112-829b-5a190f37a00f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_67872cd1-0fd0-49f7-97f6-7b95d28bfdeb" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_deb0a21b-2f95-4383-86b8-dc842c9292d2" xlink:to="loc_srt_MaximumMember_67872cd1-0fd0-49f7-97f6-7b95d28bfdeb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingAxis_dc1e6b1a-b966-426e-9320-fc414015040b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VestingAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_41efb38f-7f74-42c3-aeb1-1699aa8d5e1e" xlink:to="loc_us-gaap_VestingAxis_dc1e6b1a-b966-426e-9320-fc414015040b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_dc1e6b1a-b966-426e-9320-fc414015040b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VestingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VestingAxis_dc1e6b1a-b966-426e-9320-fc414015040b" xlink:to="loc_us-gaap_VestingDomain_dc1e6b1a-b966-426e-9320-fc414015040b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_239f13f2-ec31-43da-9cea-ee4b93ffd31d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VestingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VestingAxis_dc1e6b1a-b966-426e-9320-fc414015040b" xlink:to="loc_us-gaap_VestingDomain_239f13f2-ec31-43da-9cea-ee4b93ffd31d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ShareBasedPaymentArrangementAggregateTargetMember_019c91da-5584-7853-b062-878282897596" xlink:href="mgni-20251231.xsd#mgni_ShareBasedPaymentArrangementAggregateTargetMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_239f13f2-ec31-43da-9cea-ee4b93ffd31d" xlink:to="loc_mgni_ShareBasedPaymentArrangementAggregateTargetMember_019c91da-5584-7853-b062-878282897596" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_1707b258-cceb-41e5-9114-32b1aa4e1f63" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationAwardTrancheOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_239f13f2-ec31-43da-9cea-ee4b93ffd31d" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_1707b258-cceb-41e5-9114-32b1aa4e1f63" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheTwoMember_e6dc1cde-eba8-4c8d-a26d-274e9f7cc2d0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationAwardTrancheTwoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_239f13f2-ec31-43da-9cea-ee4b93ffd31d" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheTwoMember_e6dc1cde-eba8-4c8d-a26d-274e9f7cc2d0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheThreeMember_ea3b0e9b-75c8-4e43-87c5-591e848c6039" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationAwardTrancheThreeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_239f13f2-ec31-43da-9cea-ee4b93ffd31d" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheThreeMember_ea3b0e9b-75c8-4e43-87c5-591e848c6039" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualAxis_925f8499-99b8-4409-a295-daa90c71c876" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_TitleOfIndividualAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_41efb38f-7f74-42c3-aeb1-1699aa8d5e1e" xlink:to="loc_srt_TitleOfIndividualAxis_925f8499-99b8-4409-a295-daa90c71c876" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_925f8499-99b8-4409-a295-daa90c71c876_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_TitleOfIndividualAxis_925f8499-99b8-4409-a295-daa90c71c876" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_925f8499-99b8-4409-a295-daa90c71c876_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_e8485f1a-a764-4034-a674-5b6255a9c41c" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_TitleOfIndividualAxis_925f8499-99b8-4409-a295-daa90c71c876" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_e8485f1a-a764-4034-a674-5b6255a9c41c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ChiefExecutiveOfficerMember_e03e9489-d74a-4438-a0b9-96e5a69054a7" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ChiefExecutiveOfficerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_e8485f1a-a764-4034-a674-5b6255a9c41c" xlink:to="loc_srt_ChiefExecutiveOfficerMember_e03e9489-d74a-4438-a0b9-96e5a69054a7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_632606ef-1ede-4c6d-9419-89899a542986" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_c789e798-2e35-41e5-bdc1-59bf7ef42465" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_632606ef-1ede-4c6d-9419-89899a542986" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_NumberOfTranches_092a40fe-0d77-4a0a-a7e0-96c0f69957b9" xlink:href="mgni-20251231.xsd#mgni_NumberOfTranches"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_c789e798-2e35-41e5-bdc1-59bf7ef42465" xlink:to="loc_mgni_NumberOfTranches_092a40fe-0d77-4a0a-a7e0-96c0f69957b9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestingStockPriceTrigger_196245e9-5bea-49c2-a17e-c0cb593b5a7e" xlink:href="mgni-20251231.xsd#mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestingStockPriceTrigger"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_c789e798-2e35-41e5-bdc1-59bf7ef42465" xlink:to="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestingStockPriceTrigger_196245e9-5bea-49c2-a17e-c0cb593b5a7e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestingThresholdConsecutiveTradingDays_ae2e2ff5-c617-4c61-98ba-97f5210ecce9" xlink:href="mgni-20251231.xsd#mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestingThresholdConsecutiveTradingDays"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_c789e798-2e35-41e5-bdc1-59bf7ef42465" xlink:to="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestingThresholdConsecutiveTradingDays_ae2e2ff5-c617-4c61-98ba-97f5210ecce9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_b7c56a15-af2f-4346-9b54-dfc5193ca874" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_c789e798-2e35-41e5-bdc1-59bf7ef42465" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_b7c56a15-af2f-4346-9b54-dfc5193ca874" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardPerformanceMeasurementPercentage_1504cc62-300f-46b7-ad83-d4a7719fee67" xlink:href="mgni-20251231.xsd#mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardPerformanceMeasurementPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_c789e798-2e35-41e5-bdc1-59bf7ef42465" xlink:to="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardPerformanceMeasurementPercentage_1504cc62-300f-46b7-ad83-d4a7719fee67" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019c7634-be92-782a-be6c-ea3eb9a3b8bd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_c789e798-2e35-41e5-bdc1-59bf7ef42465" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019c7634-be92-782a-be6c-ea3eb9a3b8bd" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_49bec52a-56b6-4f25-9697-f9b18409b41d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_c789e798-2e35-41e5-bdc1-59bf7ef42465" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_49bec52a-56b6-4f25-9697-f9b18409b41d" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue_ca4c62a1-9e38-4c08-b4ae-cf7232fca054" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_c789e798-2e35-41e5-bdc1-59bf7ef42465" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue_ca4c62a1-9e38-4c08-b4ae-cf7232fca054" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueNonvested_56c9ac11-6626-4002-8677-2dd5deea9983" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueNonvested"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_c789e798-2e35-41e5-bdc1-59bf7ef42465" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardEquityInstrumentsOtherThanOptionsAggregateIntrinsicValueNonvested_56c9ac11-6626-4002-8677-2dd5deea9983" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_05f747d2-b4ef-4883-8184-40d11939580b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_c789e798-2e35-41e5-bdc1-59bf7ef42465" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_05f747d2-b4ef-4883-8184-40d11939580b" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_0d8f3f53-a696-4bc7-bf23-63d7793ed59e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_c789e798-2e35-41e5-bdc1-59bf7ef42465" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_0d8f3f53-a696-4bc7-bf23-63d7793ed59e" xlink:type="arc" order="11"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/StockBasedCompensationScheduleofPerformanceStockUnitsActivityDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#StockBasedCompensationScheduleofPerformanceStockUnitsActivityDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/StockBasedCompensationScheduleofPerformanceStockUnitsActivityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_d9c2815d-5818-4163-8a92-7a9ab3fb0bc7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_2f877f0f-9e67-48cc-ac79-cf2fa2f27ed5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_d9c2815d-5818-4163-8a92-7a9ab3fb0bc7" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_2f877f0f-9e67-48cc-ac79-cf2fa2f27ed5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_e110492b-505d-4850-a83c-966f2d8a2d39" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_2f877f0f-9e67-48cc-ac79-cf2fa2f27ed5" xlink:to="loc_us-gaap_AwardTypeAxis_e110492b-505d-4850-a83c-966f2d8a2d39" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_e110492b-505d-4850-a83c-966f2d8a2d39_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_e110492b-505d-4850-a83c-966f2d8a2d39" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_e110492b-505d-4850-a83c-966f2d8a2d39_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_0cb3fe05-49b4-41e3-aac5-c46812465751" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_e110492b-505d-4850-a83c-966f2d8a2d39" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_0cb3fe05-49b4-41e3-aac5-c46812465751" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_5fef0508-24a4-4e94-b80d-794e1e374ca6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_0cb3fe05-49b4-41e3-aac5-c46812465751" xlink:to="loc_us-gaap_PerformanceSharesMember_5fef0508-24a4-4e94-b80d-794e1e374ca6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingAxis_019c4c50-eea8-7574-b612-5cca4b15b076" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VestingAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_2f877f0f-9e67-48cc-ac79-cf2fa2f27ed5" xlink:to="loc_us-gaap_VestingAxis_019c4c50-eea8-7574-b612-5cca4b15b076" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_019c4c50-eea8-7574-b612-5cca4b15b076_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VestingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VestingAxis_019c4c50-eea8-7574-b612-5cca4b15b076" xlink:to="loc_us-gaap_VestingDomain_019c4c50-eea8-7574-b612-5cca4b15b076_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_019c4c51-23ae-7a29-9a44-096cd04d77e2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VestingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VestingAxis_019c4c50-eea8-7574-b612-5cca4b15b076" xlink:to="loc_us-gaap_VestingDomain_019c4c51-23ae-7a29-9a44-096cd04d77e2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_019c4c51-65dc-7d25-bf29-2ddd0f9d5ab9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationAwardTrancheOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_019c4c51-23ae-7a29-9a44-096cd04d77e2" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_019c4c51-65dc-7d25-bf29-2ddd0f9d5ab9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingRollForward_54728f47-9689-4362-a4e3-ae482a0985ba" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_d9c2815d-5818-4163-8a92-7a9ab3fb0bc7" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingRollForward_54728f47-9689-4362-a4e3-ae482a0985ba" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_a7d405b7-b37e-4de9-9cb7-70355f3929be" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingRollForward_54728f47-9689-4362-a4e3-ae482a0985ba" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_a7d405b7-b37e-4de9-9cb7-70355f3929be" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019c75f2-2881-7371-855a-14e55679ee0a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingRollForward_54728f47-9689-4362-a4e3-ae482a0985ba" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019c75f2-2881-7371-855a-14e55679ee0a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_019afeef-0a42-7b22-878b-4b0902729164" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingRollForward_54728f47-9689-4362-a4e3-ae482a0985ba" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_019afeef-0a42-7b22-878b-4b0902729164" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_1a46369e-87b2-461f-b624-84f13c3d4935" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_ce789cc8-6af7-44b0-a997-2b4ceb238769" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_d9c2815d-5818-4163-8a92-7a9ab3fb0bc7" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_ce789cc8-6af7-44b0-a997-2b4ceb238769" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_4c602ec3-4884-4c55-9be3-5f76bd0aee5d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_ce789cc8-6af7-44b0-a997-2b4ceb238769" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_4c602ec3-4884-4c55-9be3-5f76bd0aee5d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodIncludingGrantsAboveTarget_019c75ed-b40e-7d78-a9db-9d628221b4dd" xlink:href="mgni-20251231.xsd#mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodIncludingGrantsAboveTarget"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_ce789cc8-6af7-44b0-a997-2b4ceb238769" xlink:to="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodIncludingGrantsAboveTarget_019c75ed-b40e-7d78-a9db-9d628221b4dd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_019afef0-bd1b-7cc2-a6ff-7945412884c8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_ce789cc8-6af7-44b0-a997-2b4ceb238769" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_019afef0-bd1b-7cc2-a6ff-7945412884c8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_8a2c1e61-c354-441d-acb7-b97be0282239" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodAboveTarget_019c4c52-638a-74c9-832c-68b30e0e2ea4" xlink:href="mgni-20251231.xsd#mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodAboveTarget"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_d9c2815d-5818-4163-8a92-7a9ab3fb0bc7" xlink:to="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodAboveTarget_019c4c52-638a-74c9-832c-68b30e0e2ea4" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/StockBasedCompensationScheduleofWeightedAverageAssumptionsDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#StockBasedCompensationScheduleofWeightedAverageAssumptionsDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/StockBasedCompensationScheduleofWeightedAverageAssumptionsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6641de3a-7299-4903-b528-ba282512f24b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_74231731-b7bb-4730-a0ae-cc89c5080124" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6641de3a-7299-4903-b528-ba282512f24b" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_74231731-b7bb-4730-a0ae-cc89c5080124" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_96de4343-d65a-4e49-beac-4937ecae46bf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_74231731-b7bb-4730-a0ae-cc89c5080124" xlink:to="loc_us-gaap_AwardTypeAxis_96de4343-d65a-4e49-beac-4937ecae46bf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_96de4343-d65a-4e49-beac-4937ecae46bf_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_96de4343-d65a-4e49-beac-4937ecae46bf" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_96de4343-d65a-4e49-beac-4937ecae46bf_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_8dc1f34c-4279-4af1-afba-3a691d9a7606" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_96de4343-d65a-4e49-beac-4937ecae46bf" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_8dc1f34c-4279-4af1-afba-3a691d9a7606" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_bbba4a7d-3bb3-4fd2-aff4-ca37f2a2e50b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_8dc1f34c-4279-4af1-afba-3a691d9a7606" xlink:to="loc_us-gaap_PerformanceSharesMember_bbba4a7d-3bb3-4fd2-aff4-ca37f2a2e50b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_30b0b25d-d2fb-4dd1-9359-9d11c7e192cf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6641de3a-7299-4903-b528-ba282512f24b" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_30b0b25d-d2fb-4dd1-9359-9d11c7e192cf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_1358b6d3-4857-4733-ab24-8bb7105cfe17" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6641de3a-7299-4903-b528-ba282512f24b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_1358b6d3-4857-4733-ab24-8bb7105cfe17" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsWeightedAverageVolatilityRate_54c55d62-c890-422e-baed-b534e0f1d5b4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsWeightedAverageVolatilityRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6641de3a-7299-4903-b528-ba282512f24b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsWeightedAverageVolatilityRate_54c55d62-c890-422e-baed-b534e0f1d5b4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsSelectedPeerCompaniesWeightedAverageVolatilityRate_2262bd24-a835-4950-ba0d-dd90573b1da4" xlink:href="mgni-20251231.xsd#mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsSelectedPeerCompaniesWeightedAverageVolatilityRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6641de3a-7299-4903-b528-ba282512f24b" xlink:to="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsSelectedPeerCompaniesWeightedAverageVolatilityRate_2262bd24-a835-4950-ba0d-dd90573b1da4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsCorrelationCoefficients_7d6055cc-c9c6-4e69-9f80-0f9effe2d05f" xlink:href="mgni-20251231.xsd#mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsCorrelationCoefficients"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6641de3a-7299-4903-b528-ba282512f24b" xlink:to="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsCorrelationCoefficients_7d6055cc-c9c6-4e69-9f80-0f9effe2d05f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsSelectedPeerCompaniesCorrelationCoefficients_0a959145-45b1-444c-ab1a-0b49568c314b" xlink:href="mgni-20251231.xsd#mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsSelectedPeerCompaniesCorrelationCoefficients"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6641de3a-7299-4903-b528-ba282512f24b" xlink:to="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsSelectedPeerCompaniesCorrelationCoefficients_0a959145-45b1-444c-ab1a-0b49568c314b" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_e5e6e3f4-b38b-4858-bd6c-a14275cfd664" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_6641de3a-7299-4903-b528-ba282512f24b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_e5e6e3f4-b38b-4858-bd6c-a14275cfd664" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/StockBasedCompensationEmployeeStockPurchasePlanNarrativeDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#StockBasedCompensationEmployeeStockPurchasePlanNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/StockBasedCompensationEmployeeStockPurchasePlanNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_b439bced-d36a-404f-ae2c-6576eb4d1f07" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_294d26d4-9811-44f0-b856-e7c32de2c666" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_b439bced-d36a-404f-ae2c-6576eb4d1f07" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_294d26d4-9811-44f0-b856-e7c32de2c666" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_af898743-e26e-467a-b7e8-5bbc976a4d9c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_294d26d4-9811-44f0-b856-e7c32de2c666" xlink:to="loc_us-gaap_AwardTypeAxis_af898743-e26e-467a-b7e8-5bbc976a4d9c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_af898743-e26e-467a-b7e8-5bbc976a4d9c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_af898743-e26e-467a-b7e8-5bbc976a4d9c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_af898743-e26e-467a-b7e8-5bbc976a4d9c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_554eae92-950e-4c50-bd42-769dfb3a2131" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_af898743-e26e-467a-b7e8-5bbc976a4d9c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_554eae92-950e-4c50-bd42-769dfb3a2131" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_b0624179-4b90-440d-99b7-ed525753052e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_554eae92-950e-4c50-bd42-769dfb3a2131" xlink:to="loc_us-gaap_EmployeeStockMember_b0624179-4b90-440d-99b7-ed525753052e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_c4c550a4-2b0a-457e-ae7f-a95f45aa764f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_294d26d4-9811-44f0-b856-e7c32de2c666" xlink:to="loc_us-gaap_PlanNameAxis_c4c550a4-2b0a-457e-ae7f-a95f45aa764f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_c4c550a4-2b0a-457e-ae7f-a95f45aa764f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_c4c550a4-2b0a-457e-ae7f-a95f45aa764f" xlink:to="loc_us-gaap_PlanNameDomain_c4c550a4-2b0a-457e-ae7f-a95f45aa764f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_4c6d027d-9385-4600-a077-242cbcc31478" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_c4c550a4-2b0a-457e-ae7f-a95f45aa764f" xlink:to="loc_us-gaap_PlanNameDomain_4c6d027d-9385-4600-a077-242cbcc31478" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_A2014EmployeeStockPurchasePlanMember_ed4242dc-6604-4869-ac48-30a81684712c" xlink:href="mgni-20251231.xsd#mgni_A2014EmployeeStockPurchasePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_4c6d027d-9385-4600-a077-242cbcc31478" xlink:to="loc_mgni_A2014EmployeeStockPurchasePlanMember_ed4242dc-6604-4869-ac48-30a81684712c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumEmployeeSubscriptionRate_90f2933f-ba03-43e3-abd7-b5ba754d4a4f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumEmployeeSubscriptionRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_b439bced-d36a-404f-ae2c-6576eb4d1f07" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumEmployeeSubscriptionRate_90f2933f-ba03-43e3-abd7-b5ba754d4a4f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardOfferingPeriod_fcee0c04-bd0b-4550-88cc-d65200cc12cc" xlink:href="mgni-20251231.xsd#mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardOfferingPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_b439bced-d36a-404f-ae2c-6576eb4d1f07" xlink:to="loc_mgni_ShareBasedCompensationArrangementByShareBasedPaymentAwardOfferingPeriod_fcee0c04-bd0b-4550-88cc-d65200cc12cc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent_9b010cfd-c941-4fc9-b4a3-e8d91e011e91" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_b439bced-d36a-404f-ae2c-6576eb4d1f07" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardPurchasePriceOfCommonStockPercent_9b010cfd-c941-4fc9-b4a3-e8d91e011e91" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_488baefd-a554-4ce8-959a-fc8c1911cd31" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_b439bced-d36a-404f-ae2c-6576eb4d1f07" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_488baefd-a554-4ce8-959a-fc8c1911cd31" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/StockBasedCompensationScheduleofStockBasedCompensationExpenseDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#StockBasedCompensationScheduleofStockBasedCompensationExpenseDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/StockBasedCompensationScheduleofStockBasedCompensationExpenseDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_b849ada6-698e-42c8-838e-36429206e746" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_2f95dcd4-cf1b-4e56-b23e-8c96f2fad026" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_b849ada6-698e-42c8-838e-36429206e746" xlink:to="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_2f95dcd4-cf1b-4e56-b23e-8c96f2fad026" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_7992eb95-a8e7-4b90-a189-d263b1bc256d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_2f95dcd4-cf1b-4e56-b23e-8c96f2fad026" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_7992eb95-a8e7-4b90-a189-d263b1bc256d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_7992eb95-a8e7-4b90-a189-d263b1bc256d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_7992eb95-a8e7-4b90-a189-d263b1bc256d" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_7992eb95-a8e7-4b90-a189-d263b1bc256d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_2c23d2cf-ccd1-45d9-a4aa-c6b8f495560e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_7992eb95-a8e7-4b90-a189-d263b1bc256d" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_2c23d2cf-ccd1-45d9-a4aa-c6b8f495560e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfSalesMember_b9363a1f-742f-46cd-8511-19251e0a6e6b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CostOfSalesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_2c23d2cf-ccd1-45d9-a4aa-c6b8f495560e" xlink:to="loc_us-gaap_CostOfSalesMember_b9363a1f-742f-46cd-8511-19251e0a6e6b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingAndMarketingExpenseMember_c88964ab-0c27-4425-8516-7c614cecf1e1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SellingAndMarketingExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_2c23d2cf-ccd1-45d9-a4aa-c6b8f495560e" xlink:to="loc_us-gaap_SellingAndMarketingExpenseMember_c88964ab-0c27-4425-8516-7c614cecf1e1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpenseMember_eecd51e8-b60a-4b5d-9873-31f956ff642d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResearchAndDevelopmentExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_2c23d2cf-ccd1-45d9-a4aa-c6b8f495560e" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpenseMember_eecd51e8-b60a-4b5d-9873-31f956ff642d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GeneralAndAdministrativeExpenseMember_6a104707-1413-4d27-9c0d-b696b0700361" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GeneralAndAdministrativeExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_2c23d2cf-ccd1-45d9-a4aa-c6b8f495560e" xlink:to="loc_us-gaap_GeneralAndAdministrativeExpenseMember_6a104707-1413-4d27-9c0d-b696b0700361" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringChargesMember_a7a13a48-9cf8-4419-a3f8-26f8c6bd2023" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestructuringChargesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_2c23d2cf-ccd1-45d9-a4aa-c6b8f495560e" xlink:to="loc_us-gaap_RestructuringChargesMember_a7a13a48-9cf8-4419-a3f8-26f8c6bd2023" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_7dbe8963-8344-4b03-a84e-3e66b1434b7a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_b849ada6-698e-42c8-838e-36429206e746" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_7dbe8963-8344-4b03-a84e-3e66b1434b7a" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/IncomeTaxesNarrativeDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#IncomeTaxesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/IncomeTaxesNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLossCarryforwardsLineItems_13a17418-a8d3-4b87-979d-5c634f843030" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLossCarryforwardsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLossCarryforwardsTable_547caeb1-2a83-4c3d-8649-a71837a288c7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLossCarryforwardsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_13a17418-a8d3-4b87-979d-5c634f843030" xlink:to="loc_us-gaap_OperatingLossCarryforwardsTable_547caeb1-2a83-4c3d-8649-a71837a288c7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityAxis_ea9f82db-09e3-447b-bda6-34e440a30499" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OperatingLossCarryforwardsTable_547caeb1-2a83-4c3d-8649-a71837a288c7" xlink:to="loc_us-gaap_IncomeTaxAuthorityAxis_ea9f82db-09e3-447b-bda6-34e440a30499" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_ea9f82db-09e3-447b-bda6-34e440a30499_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_ea9f82db-09e3-447b-bda6-34e440a30499" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_ea9f82db-09e3-447b-bda6-34e440a30499_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_456c47f8-340c-459a-ae74-260d823f6030" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_ea9f82db-09e3-447b-bda6-34e440a30499" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_456c47f8-340c-459a-ae74-260d823f6030" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DomesticCountryMember_67c78aff-df76-4f03-b925-b8f837f7fe09" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DomesticCountryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_456c47f8-340c-459a-ae74-260d823f6030" xlink:to="loc_us-gaap_DomesticCountryMember_67c78aff-df76-4f03-b925-b8f837f7fe09" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StateAndLocalJurisdictionMember_b73ac4f3-2f77-4518-9526-e82f58763097" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StateAndLocalJurisdictionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_456c47f8-340c-459a-ae74-260d823f6030" xlink:to="loc_us-gaap_StateAndLocalJurisdictionMember_b73ac4f3-2f77-4518-9526-e82f58763097" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignCountryMember_e01e3ee4-784f-4984-b128-7a718747e850" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ForeignCountryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_456c47f8-340c-459a-ae74-260d823f6030" xlink:to="loc_us-gaap_ForeignCountryMember_e01e3ee4-784f-4984-b128-7a718747e850" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TaxCreditCarryforwardAxis_118165e6-f5bf-4978-9bf0-f2b4b0adc531" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TaxCreditCarryforwardAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OperatingLossCarryforwardsTable_547caeb1-2a83-4c3d-8649-a71837a288c7" xlink:to="loc_us-gaap_TaxCreditCarryforwardAxis_118165e6-f5bf-4978-9bf0-f2b4b0adc531" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TaxCreditCarryforwardNameDomain_118165e6-f5bf-4978-9bf0-f2b4b0adc531_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TaxCreditCarryforwardNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TaxCreditCarryforwardAxis_118165e6-f5bf-4978-9bf0-f2b4b0adc531" xlink:to="loc_us-gaap_TaxCreditCarryforwardNameDomain_118165e6-f5bf-4978-9bf0-f2b4b0adc531_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TaxCreditCarryforwardNameDomain_b3339e9a-7d86-4273-8038-079b67730a18" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TaxCreditCarryforwardNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TaxCreditCarryforwardAxis_118165e6-f5bf-4978-9bf0-f2b4b0adc531" xlink:to="loc_us-gaap_TaxCreditCarryforwardNameDomain_b3339e9a-7d86-4273-8038-079b67730a18" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchMember_e233397d-7da2-49e9-9a98-1ca7c097ec9e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResearchMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TaxCreditCarryforwardNameDomain_b3339e9a-7d86-4273-8038-079b67730a18" xlink:to="loc_us-gaap_ResearchMember_e233397d-7da2-49e9-9a98-1ca7c097ec9e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_cf1530e0-17b4-4e82-85cc-49c3adb6df14" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_OperatingLossCarryforwardsTable_547caeb1-2a83-4c3d-8649-a71837a288c7" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_cf1530e0-17b4-4e82-85cc-49c3adb6df14" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_cf1530e0-17b4-4e82-85cc-49c3adb6df14_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_cf1530e0-17b4-4e82-85cc-49c3adb6df14" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_cf1530e0-17b4-4e82-85cc-49c3adb6df14_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_d917436e-c9e0-48d9-83e3-d04fb530987c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_cf1530e0-17b4-4e82-85cc-49c3adb6df14" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_d917436e-c9e0-48d9-83e3-d04fb530987c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNoncurrentLiabilitiesMember_cb2e278a-566d-45c5-b800-a054b7e1eb06" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherNoncurrentLiabilitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_d917436e-c9e0-48d9-83e3-d04fb530987c" xlink:to="loc_us-gaap_OtherNoncurrentLiabilitiesMember_cb2e278a-566d-45c5-b800-a054b7e1eb06" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNoncurrentAssetsMember_49c6a7f9-6be4-4b26-9bb9-bb14af825bd5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherNoncurrentAssetsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_d917436e-c9e0-48d9-83e3-d04fb530987c" xlink:to="loc_us-gaap_OtherNoncurrentAssetsMember_49c6a7f9-6be4-4b26-9bb9-bb14af825bd5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_d3d4e7e5-5a46-4a41-a940-34668cfe4326" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_13a17418-a8d3-4b87-979d-5c634f843030" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_d3d4e7e5-5a46-4a41-a940-34668cfe4326" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxAssetsLiabilitiesNet_019c4cfa-cdb9-726e-87f4-1b4fd8656c39" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredTaxAssetsLiabilitiesNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_13a17418-a8d3-4b87-979d-5c634f843030" xlink:to="loc_us-gaap_DeferredTaxAssetsLiabilitiesNet_019c4cfa-cdb9-726e-87f4-1b4fd8656c39" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxAssetsNet_019c7659-931c-7d31-8796-f5b739ce2f45" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredTaxAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_13a17418-a8d3-4b87-979d-5c634f843030" xlink:to="loc_us-gaap_DeferredTaxAssetsNet_019c7659-931c-7d31-8796-f5b739ce2f45" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxLiabilities_8db56f56-91cd-4c19-b09f-ca5c89e64826" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredTaxLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_13a17418-a8d3-4b87-979d-5c634f843030" xlink:to="loc_us-gaap_DeferredTaxLiabilities_8db56f56-91cd-4c19-b09f-ca5c89e64826" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount_c55b9155-0a74-4efc-87b5-aeef7b7ff45a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_13a17418-a8d3-4b87-979d-5c634f843030" xlink:to="loc_us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount_c55b9155-0a74-4efc-87b5-aeef7b7ff45a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLossCarryforwards_0ec18166-7a5e-405c-9568-1aa89202d2d8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLossCarryforwards"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_13a17418-a8d3-4b87-979d-5c634f843030" xlink:to="loc_us-gaap_OperatingLossCarryforwards_0ec18166-7a5e-405c-9568-1aa89202d2d8" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TaxCreditCarryforwardAmount_097bc181-edd8-4bd3-8d30-10571105b1ac" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TaxCreditCarryforwardAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_13a17418-a8d3-4b87-979d-5c634f843030" xlink:to="loc_us-gaap_TaxCreditCarryforwardAmount_097bc181-edd8-4bd3-8d30-10571105b1ac" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UndistributedEarningsOfForeignSubsidiaries_33674b21-da6a-4c99-afea-9f91bf05e051" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_UndistributedEarningsOfForeignSubsidiaries"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingLossCarryforwardsLineItems_13a17418-a8d3-4b87-979d-5c634f843030" xlink:to="loc_us-gaap_UndistributedEarningsOfForeignSubsidiaries_33674b21-da6a-4c99-afea-9f91bf05e051" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/IncomeTaxesScheduleofUSFederalStatutoryIncomeTaxRateDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#IncomeTaxesScheduleofUSFederalStatutoryIncomeTaxRateDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/IncomeTaxesScheduleofUSFederalStatutoryIncomeTaxRateDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationLineItems_acf75cab-752a-4b71-a0c2-f2b608a009a6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationTable_7150fedd-0c28-4afc-8d9b-913ad4e72dae" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateReconciliationLineItems_acf75cab-752a-4b71-a0c2-f2b608a009a6" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateReconciliationTable_7150fedd-0c28-4afc-8d9b-913ad4e72dae" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityAxis_cab5930e-30bb-4c3f-ab85-189ea9d17555" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateReconciliationTable_7150fedd-0c28-4afc-8d9b-913ad4e72dae" xlink:to="loc_us-gaap_IncomeTaxAuthorityAxis_cab5930e-30bb-4c3f-ab85-189ea9d17555" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_cab5930e-30bb-4c3f-ab85-189ea9d17555_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_cab5930e-30bb-4c3f-ab85-189ea9d17555" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_cab5930e-30bb-4c3f-ab85-189ea9d17555_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_25b17d1b-b03c-4bb3-92a5-ae048d6d49fb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_cab5930e-30bb-4c3f-ab85-189ea9d17555" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_25b17d1b-b03c-4bb3-92a5-ae048d6d49fb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignCountryMember_a2915750-248e-4c68-8c3a-32ddace33707" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ForeignCountryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_25b17d1b-b03c-4bb3-92a5-ae048d6d49fb" xlink:to="loc_us-gaap_ForeignCountryMember_a2915750-248e-4c68-8c3a-32ddace33707" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_GB_eb0a7d17-36a2-4ffe-b968-6237c4f86771" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_GB"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ForeignCountryMember_a2915750-248e-4c68-8c3a-32ddace33707" xlink:to="loc_country_GB_eb0a7d17-36a2-4ffe-b968-6237c4f86771" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DomesticCountryMember_86326800-692f-41f9-a96f-f0d5a168b936" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DomesticCountryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_25b17d1b-b03c-4bb3-92a5-ae048d6d49fb" xlink:to="loc_us-gaap_DomesticCountryMember_86326800-692f-41f9-a96f-f0d5a168b936" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_GB_623077d3-2c83-4bdb-9867-50641efcc1b6" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_GB"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StateAndLocalJurisdictionMember_019c7616-6d57-7093-96ef-c60a5408b1c1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StateAndLocalJurisdictionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DomesticCountryMember_86326800-692f-41f9-a96f-f0d5a168b936" xlink:to="loc_us-gaap_StateAndLocalJurisdictionMember_019c7616-6d57-7093-96ef-c60a5408b1c1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_019c7618-6d31-7b37-8ab0-e6a39d5531b2" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DomesticCountryMember_86326800-692f-41f9-a96f-f0d5a168b936" xlink:to="loc_country_US_019c7618-6d31-7b37-8ab0-e6a39d5531b2" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract_58c08172-4a8b-4d38-97c7-1e241367c74e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateReconciliationLineItems_acf75cab-752a-4b71-a0c2-f2b608a009a6" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract_58c08172-4a8b-4d38-97c7-1e241367c74e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate_f6c024eb-8c17-4060-a651-a4714b949ab0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract_58c08172-4a8b-4d38-97c7-1e241367c74e" xlink:to="loc_us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate_f6c024eb-8c17-4060-a651-a4714b949ab0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes_90fee938-1b5a-4d8e-85ba-5f1afc9c6d6d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract_58c08172-4a8b-4d38-97c7-1e241367c74e" xlink:to="loc_us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes_90fee938-1b5a-4d8e-85ba-5f1afc9c6d6d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance_b41add93-f0ea-4568-ac9e-a78d92b13693" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract_58c08172-4a8b-4d38-97c7-1e241367c74e" xlink:to="loc_us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance_b41add93-f0ea-4568-ac9e-a78d92b13693" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_EffectiveIncomeTaxRateReconciliationStockOptionWindfallBenefitAmount_019c4c68-2aab-7b74-a34a-bca9a43c21e2" xlink:href="mgni-20251231.xsd#mgni_EffectiveIncomeTaxRateReconciliationStockOptionWindfallBenefitAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract_58c08172-4a8b-4d38-97c7-1e241367c74e" xlink:to="loc_mgni_EffectiveIncomeTaxRateReconciliationStockOptionWindfallBenefitAmount_019c4c68-2aab-7b74-a34a-bca9a43c21e2" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxReconciliationOtherReconcilingItems_5e0c6802-2534-4609-a595-6ae6bb10946f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationOtherReconcilingItems"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract_58c08172-4a8b-4d38-97c7-1e241367c74e" xlink:to="loc_us-gaap_IncomeTaxReconciliationOtherReconcilingItems_5e0c6802-2534-4609-a595-6ae6bb10946f" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxReconciliationForeignIncomeTaxRateDifferential_157be7c5-a1c2-4986-88ba-9dfa80931785" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationForeignIncomeTaxRateDifferential"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract_58c08172-4a8b-4d38-97c7-1e241367c74e" xlink:to="loc_us-gaap_IncomeTaxReconciliationForeignIncomeTaxRateDifferential_157be7c5-a1c2-4986-88ba-9dfa80931785" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxReconciliationTaxCreditsResearch_e2d71558-b3cc-412e-ad1d-aa647cdeb670" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationTaxCreditsResearch"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract_58c08172-4a8b-4d38-97c7-1e241367c74e" xlink:to="loc_us-gaap_IncomeTaxReconciliationTaxCreditsResearch_e2d71558-b3cc-412e-ad1d-aa647cdeb670" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_EffectiveIncomeTaxRateReconciliationOfficersCompensationAmount_019afed2-9a09-73f8-a30e-e26024e68799" xlink:href="mgni-20251231.xsd#mgni_EffectiveIncomeTaxRateReconciliationOfficersCompensationAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract_58c08172-4a8b-4d38-97c7-1e241367c74e" xlink:to="loc_mgni_EffectiveIncomeTaxRateReconciliationOfficersCompensationAmount_019afed2-9a09-73f8-a30e-e26024e68799" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxReconciliationNondeductibleExpenseShareBasedCompensationCost_71450bc6-7253-4f71-babd-52bc1e9734ef" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationNondeductibleExpenseShareBasedCompensationCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract_58c08172-4a8b-4d38-97c7-1e241367c74e" xlink:to="loc_us-gaap_IncomeTaxReconciliationNondeductibleExpenseShareBasedCompensationCost_71450bc6-7253-4f71-babd-52bc1e9734ef" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxReconciliationNondeductibleExpenseOther_5ea9f0b3-bec2-4942-bfed-74fed1639a6f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationNondeductibleExpenseOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract_58c08172-4a8b-4d38-97c7-1e241367c74e" xlink:to="loc_us-gaap_IncomeTaxReconciliationNondeductibleExpenseOther_5ea9f0b3-bec2-4942-bfed-74fed1639a6f" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxReconciliationPriorYearIncomeTaxes_d21999a9-0d1c-4b03-9b27-51490d36165a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationPriorYearIncomeTaxes"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract_58c08172-4a8b-4d38-97c7-1e241367c74e" xlink:to="loc_us-gaap_IncomeTaxReconciliationPriorYearIncomeTaxes_d21999a9-0d1c-4b03-9b27-51490d36165a" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxReconciliationTaxContingenciesOther_09bbd19a-085c-4c79-88e8-2743f645f8af" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxReconciliationTaxContingenciesOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract_58c08172-4a8b-4d38-97c7-1e241367c74e" xlink:to="loc_us-gaap_IncomeTaxReconciliationTaxContingenciesOther_09bbd19a-085c-4c79-88e8-2743f645f8af" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_b1b4ef4b-2aa5-4030-9cb4-eebc57e10a9e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxExpenseBenefitContinuingOperationsIncomeTaxReconciliationAbstract_58c08172-4a8b-4d38-97c7-1e241367c74e" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_b1b4ef4b-2aa5-4030-9cb4-eebc57e10a9e" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract_64405e61-169a-45f7-8e4b-2c98139d04e0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateReconciliationLineItems_acf75cab-752a-4b71-a0c2-f2b608a009a6" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract_64405e61-169a-45f7-8e4b-2c98139d04e0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate_170defef-7538-4704-aa61-44d2ce56ddb2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract_64405e61-169a-45f7-8e4b-2c98139d04e0" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate_170defef-7538-4704-aa61-44d2ce56ddb2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes_6fb56723-e884-4440-aa7a-58461c671452" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract_64405e61-169a-45f7-8e4b-2c98139d04e0" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateReconciliationStateAndLocalIncomeTaxes_6fb56723-e884-4440-aa7a-58461c671452" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance_67441b1a-9225-4d3f-845a-bb534f420281" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract_64405e61-169a-45f7-8e4b-2c98139d04e0" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateReconciliationChangeInDeferredTaxAssetsValuationAllowance_67441b1a-9225-4d3f-845a-bb534f420281" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_EffectiveIncomeTaxRateReconciliationStockOptionWindfallBenefitPercent_019c4c67-59bb-75ab-b7ad-a36fb86840ab" xlink:href="mgni-20251231.xsd#mgni_EffectiveIncomeTaxRateReconciliationStockOptionWindfallBenefitPercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract_64405e61-169a-45f7-8e4b-2c98139d04e0" xlink:to="loc_mgni_EffectiveIncomeTaxRateReconciliationStockOptionWindfallBenefitPercent_019c4c67-59bb-75ab-b7ad-a36fb86840ab" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationOtherReconcilingItemsPercent_5414567d-b592-4c64-af80-b734f5f3724d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationOtherReconcilingItemsPercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract_64405e61-169a-45f7-8e4b-2c98139d04e0" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateReconciliationOtherReconcilingItemsPercent_5414567d-b592-4c64-af80-b734f5f3724d" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationForeignIncomeTaxRateDifferential_76d1fb0a-8ea6-48e7-b10b-da3b48954262" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationForeignIncomeTaxRateDifferential"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract_64405e61-169a-45f7-8e4b-2c98139d04e0" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateReconciliationForeignIncomeTaxRateDifferential_76d1fb0a-8ea6-48e7-b10b-da3b48954262" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationTaxCreditsResearch_e40379b2-b068-4d8d-8faa-bfbb5c664282" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationTaxCreditsResearch"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract_64405e61-169a-45f7-8e4b-2c98139d04e0" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateReconciliationTaxCreditsResearch_e40379b2-b068-4d8d-8faa-bfbb5c664282" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_EffectiveIncomeTaxRateReconciliationOfficersCompensationPercent_019c57ad-b725-7800-8c7c-fbc97381c9e1" xlink:href="mgni-20251231.xsd#mgni_EffectiveIncomeTaxRateReconciliationOfficersCompensationPercent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract_64405e61-169a-45f7-8e4b-2c98139d04e0" xlink:to="loc_mgni_EffectiveIncomeTaxRateReconciliationOfficersCompensationPercent_019c57ad-b725-7800-8c7c-fbc97381c9e1" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseShareBasedCompensationCost_0a035b2b-388d-4057-9dd9-7bea1324c646" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseShareBasedCompensationCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract_64405e61-169a-45f7-8e4b-2c98139d04e0" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseShareBasedCompensationCost_0a035b2b-388d-4057-9dd9-7bea1324c646" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseMealsAndEntertainment_829fe4a0-dd37-4ff3-9a76-3a228849d476" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseMealsAndEntertainment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract_64405e61-169a-45f7-8e4b-2c98139d04e0" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseMealsAndEntertainment_829fe4a0-dd37-4ff3-9a76-3a228849d476" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseOther_42efb24e-74ed-4d30-93ad-c6b31f5fd571" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract_64405e61-169a-45f7-8e4b-2c98139d04e0" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseOther_42efb24e-74ed-4d30-93ad-c6b31f5fd571" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationPriorYearIncomeTaxes_aee37f8d-7228-49c5-9c6c-872b3ff907bc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationPriorYearIncomeTaxes"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract_64405e61-169a-45f7-8e4b-2c98139d04e0" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateReconciliationPriorYearIncomeTaxes_aee37f8d-7228-49c5-9c6c-872b3ff907bc" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationTaxContingenciesOther_8860a44b-6708-4481-beee-bd6a2b5cb987" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationTaxContingenciesOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract_64405e61-169a-45f7-8e4b-2c98139d04e0" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateReconciliationTaxContingenciesOther_8860a44b-6708-4481-beee-bd6a2b5cb987" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_EffectiveIncomeTaxRateReconciliationReturnAdjustmentProvision_019b212e-9b90-7374-9b07-0bc23033457e" xlink:href="mgni-20251231.xsd#mgni_EffectiveIncomeTaxRateReconciliationReturnAdjustmentProvision"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract_64405e61-169a-45f7-8e4b-2c98139d04e0" xlink:to="loc_mgni_EffectiveIncomeTaxRateReconciliationReturnAdjustmentProvision_019b212e-9b90-7374-9b07-0bc23033457e" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationRepatriationOfForeignEarnings_019b212e-d54d-7ec2-99fe-c63bab14d1d1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationRepatriationOfForeignEarnings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract_64405e61-169a-45f7-8e4b-2c98139d04e0" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateReconciliationRepatriationOfForeignEarnings_019b212e-d54d-7ec2-99fe-c63bab14d1d1" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperations_d35ed70f-f595-4998-af99-540e6b1ef76d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateContinuingOperations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperationsTaxRateReconciliationAbstract_64405e61-169a-45f7-8e4b-2c98139d04e0" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperations_d35ed70f-f595-4998-af99-540e6b1ef76d" xlink:type="arc" order="16"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/IncomeTaxesScheduleofIncomeTaxesPaidbyRefundsDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#IncomeTaxesScheduleofIncomeTaxesPaidbyRefundsDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/IncomeTaxesScheduleofIncomeTaxesPaidbyRefundsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationLineItems_22261980-a2dd-42bf-9ba2-844cd71e5b45" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationTable_a576b2e4-542c-4b8e-946a-d6c413451b1c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateReconciliationLineItems_22261980-a2dd-42bf-9ba2-844cd71e5b45" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateReconciliationTable_a576b2e4-542c-4b8e-946a-d6c413451b1c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityAxis_405e0737-98ed-4bbc-b265-e082444e263d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateReconciliationTable_a576b2e4-542c-4b8e-946a-d6c413451b1c" xlink:to="loc_us-gaap_IncomeTaxAuthorityAxis_405e0737-98ed-4bbc-b265-e082444e263d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_405e0737-98ed-4bbc-b265-e082444e263d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_405e0737-98ed-4bbc-b265-e082444e263d" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_405e0737-98ed-4bbc-b265-e082444e263d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_57f0f9db-45d2-45bb-91a7-bae43d3034b2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxAuthorityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_405e0737-98ed-4bbc-b265-e082444e263d" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_57f0f9db-45d2-45bb-91a7-bae43d3034b2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_stpr_NY_019c5763-5811-7408-9cf8-9291917bc1be" xlink:href="https://xbrl.sec.gov/stpr/2025/stpr-2025.xsd#stpr_NY"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_57f0f9db-45d2-45bb-91a7-bae43d3034b2" xlink:to="loc_stpr_NY_019c5763-5811-7408-9cf8-9291917bc1be" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_stpr_VA_019c5763-cea4-7048-ae79-ddb01628546d" xlink:href="https://xbrl.sec.gov/stpr/2025/stpr-2025.xsd#stpr_VA"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_57f0f9db-45d2-45bb-91a7-bae43d3034b2" xlink:to="loc_stpr_VA_019c5763-cea4-7048-ae79-ddb01628546d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_stpr_MI_019c5764-6258-76d8-9ef3-b983643b6671" xlink:href="https://xbrl.sec.gov/stpr/2025/stpr-2025.xsd#stpr_MI"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_57f0f9db-45d2-45bb-91a7-bae43d3034b2" xlink:to="loc_stpr_MI_019c5764-6258-76d8-9ef3-b983643b6671" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_AU_efbe67f6-5a65-4775-90b4-9da186ce201b" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_AU"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_57f0f9db-45d2-45bb-91a7-bae43d3034b2" xlink:to="loc_country_AU_efbe67f6-5a65-4775-90b4-9da186ce201b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_IN_019c5750-b616-76fd-a63b-43873eb5f5ab" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_IN"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_57f0f9db-45d2-45bb-91a7-bae43d3034b2" xlink:to="loc_country_IN_019c5750-b616-76fd-a63b-43873eb5f5ab" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ForeignTaxJurisdictionOtherMember_bf362009-4265-4e62-a81f-50ebf419ffa0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ForeignTaxJurisdictionOtherMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_57f0f9db-45d2-45bb-91a7-bae43d3034b2" xlink:to="loc_us-gaap_ForeignTaxJurisdictionOtherMember_bf362009-4265-4e62-a81f-50ebf419ffa0" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxPaidFederalAfterRefundReceived_eae8feb1-6675-4ae5-962c-9653d7c452fc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxPaidFederalAfterRefundReceived"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateReconciliationLineItems_22261980-a2dd-42bf-9ba2-844cd71e5b45" xlink:to="loc_us-gaap_IncomeTaxPaidFederalAfterRefundReceived_eae8feb1-6675-4ae5-962c-9653d7c452fc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxPaidStateAndLocalAfterRefundReceived_11498c3a-30ee-4e14-8ed0-af843c796f5e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxPaidStateAndLocalAfterRefundReceived"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateReconciliationLineItems_22261980-a2dd-42bf-9ba2-844cd71e5b45" xlink:to="loc_us-gaap_IncomeTaxPaidStateAndLocalAfterRefundReceived_11498c3a-30ee-4e14-8ed0-af843c796f5e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxPaidForeignAfterRefundReceived_3a380659-d388-4710-bfeb-eb6b1277bd3b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxPaidForeignAfterRefundReceived"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateReconciliationLineItems_22261980-a2dd-42bf-9ba2-844cd71e5b45" xlink:to="loc_us-gaap_IncomeTaxPaidForeignAfterRefundReceived_3a380659-d388-4710-bfeb-eb6b1277bd3b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxesPaidNet_019b99e8-03da-7741-80fc-4a2faf5fb8f7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxesPaidNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EffectiveIncomeTaxRateReconciliationLineItems_22261980-a2dd-42bf-9ba2-844cd71e5b45" xlink:to="loc_us-gaap_IncomeTaxesPaidNet_019b99e8-03da-7741-80fc-4a2faf5fb8f7" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/MergerAcquisitionandRestructuringCostsNarrativeDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#MergerAcquisitionandRestructuringCostsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/MergerAcquisitionandRestructuringCostsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_019c7e90-01be-7113-9a89-5a4b40a7c9f0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_019c7e8e-ccef-7875-a17d-a45ca19f0842" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfRestructuringAndRelatedCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019c7e90-01be-7113-9a89-5a4b40a7c9f0" xlink:to="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_019c7e8e-ccef-7875-a17d-a45ca19f0842" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_019c7e8f-156d-71fc-baa9-95a7a57f73e6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_019c7e8e-ccef-7875-a17d-a45ca19f0842" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_019c7e8f-156d-71fc-baa9-95a7a57f73e6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019c7e8f-156d-71fc-baa9-95a7a57f73e6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_019c7e8f-156d-71fc-baa9-95a7a57f73e6" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019c7e8f-156d-71fc-baa9-95a7a57f73e6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019c7e8f-5adc-74cf-a9e4-eaf874b50038" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_019c7e8f-156d-71fc-baa9-95a7a57f73e6" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019c7e8f-5adc-74cf-a9e4-eaf874b50038" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_SpotXAndTelariaMember_019c7e8f-a63f-71c5-a3a7-fd18eb81fe4e" xlink:href="mgni-20251231.xsd#mgni_SpotXAndTelariaMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019c7e8f-5adc-74cf-a9e4-eaf874b50038" xlink:to="loc_mgni_SpotXAndTelariaMember_019c7e8f-a63f-71c5-a3a7-fd18eb81fe4e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_RestructuringChargesAndBusinessCombinationAcquisitionRelatedCosts_019bfe5a-2bfe-7405-ab33-f8eb07623a51" xlink:href="mgni-20251231.xsd#mgni_RestructuringChargesAndBusinessCombinationAcquisitionRelatedCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019c7e90-01be-7113-9a89-5a4b40a7c9f0" xlink:to="loc_mgni_RestructuringChargesAndBusinessCombinationAcquisitionRelatedCosts_019bfe5a-2bfe-7405-ab33-f8eb07623a51" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_RestructuringReserveAndBusinessCombinationAcquisitionRelatedCosts_019c7e92-f0d8-706c-9a1f-1759295b4e08" xlink:href="mgni-20251231.xsd#mgni_RestructuringReserveAndBusinessCombinationAcquisitionRelatedCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019c7e90-01be-7113-9a89-5a4b40a7c9f0" xlink:to="loc_mgni_RestructuringReserveAndBusinessCombinationAcquisitionRelatedCosts_019c7e92-f0d8-706c-9a1f-1759295b4e08" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/MergerAcquisitionandRestructuringCostsScheduleofMergerandRestructuringCostActivityDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#MergerAcquisitionandRestructuringCostsScheduleofMergerandRestructuringCostActivityDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/MergerAcquisitionandRestructuringCostsScheduleofMergerandRestructuringCostActivityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostAndReserveLineItems_4808ca70-6132-4aa3-995f-3235d0ad39c1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestructuringCostAndReserveLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_8d00dd00-b527-47bd-abb1-172c50b6176d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfRestructuringAndRelatedCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_4808ca70-6132-4aa3-995f-3235d0ad39c1" xlink:to="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_8d00dd00-b527-47bd-abb1-172c50b6176d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostAndReserveAxis_18a89beb-b818-408d-912c-e73b068160fc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestructuringCostAndReserveAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_8d00dd00-b527-47bd-abb1-172c50b6176d" xlink:to="loc_us-gaap_RestructuringCostAndReserveAxis_18a89beb-b818-408d-912c-e73b068160fc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_18a89beb-b818-408d-912c-e73b068160fc_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TypeOfRestructuringDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_18a89beb-b818-408d-912c-e73b068160fc" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_18a89beb-b818-408d-912c-e73b068160fc_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_f2ac272d-be2d-4d3e-9199-179a6a823e83" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TypeOfRestructuringDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_18a89beb-b818-408d-912c-e73b068160fc" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_f2ac272d-be2d-4d3e-9199-179a6a823e83" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_EmployeeSeveranceAndOneTimeTerminationBenefitsMember_0f4b877b-067a-4ee1-9fa7-50d08b93c272" xlink:href="mgni-20251231.xsd#mgni_EmployeeSeveranceAndOneTimeTerminationBenefitsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_f2ac272d-be2d-4d3e-9199-179a6a823e83" xlink:to="loc_mgni_EmployeeSeveranceAndOneTimeTerminationBenefitsMember_0f4b877b-067a-4ee1-9fa7-50d08b93c272" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_ExitCostsMember_2d17a4c6-a599-4081-922e-78603c69d590" xlink:href="mgni-20251231.xsd#mgni_ExitCostsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_f2ac272d-be2d-4d3e-9199-179a6a823e83" xlink:to="loc_mgni_ExitCostsMember_2d17a4c6-a599-4081-922e-78603c69d590" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCharges_4baa0997-321b-4805-a725-857321f33109" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestructuringCharges"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_4808ca70-6132-4aa3-995f-3235d0ad39c1" xlink:to="loc_us-gaap_RestructuringCharges_4baa0997-321b-4805-a725-857321f33109" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_RestructuringChargesMergerAndAcquisitionAssumedLossContracts_0d386d9f-2edb-4369-a512-d714bbc56e06" xlink:href="mgni-20251231.xsd#mgni_RestructuringChargesMergerAndAcquisitionAssumedLossContracts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_4808ca70-6132-4aa3-995f-3235d0ad39c1" xlink:to="loc_mgni_RestructuringChargesMergerAndAcquisitionAssumedLossContracts_0d386d9f-2edb-4369-a512-d714bbc56e06" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_RestructuringImpairmentCosts_2d2fea03-25f6-4537-8bb0-2d612de7e494" xlink:href="mgni-20251231.xsd#mgni_RestructuringImpairmentCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_4808ca70-6132-4aa3-995f-3235d0ad39c1" xlink:to="loc_mgni_RestructuringImpairmentCosts_2d2fea03-25f6-4537-8bb0-2d612de7e494" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_RestructuringReserveAndMergerRelatedCostsSettledWithoutCash_a07922c1-1a7a-48f3-8e2c-6c78dd7b0288" xlink:href="mgni-20251231.xsd#mgni_RestructuringReserveAndMergerRelatedCostsSettledWithoutCash"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_4808ca70-6132-4aa3-995f-3235d0ad39c1" xlink:to="loc_mgni_RestructuringReserveAndMergerRelatedCostsSettledWithoutCash_a07922c1-1a7a-48f3-8e2c-6c78dd7b0288" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationAcquisitionRelatedCosts_33425f0b-03cc-4404-a796-d9db5b2b437d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationAcquisitionRelatedCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_4808ca70-6132-4aa3-995f-3235d0ad39c1" xlink:to="loc_us-gaap_BusinessCombinationAcquisitionRelatedCosts_33425f0b-03cc-4404-a796-d9db5b2b437d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_RestructuringChargesAndBusinessCombinationAcquisitionRelatedCosts_a7acf227-6c5e-45bc-824d-b0371874e37e" xlink:href="mgni-20251231.xsd#mgni_RestructuringChargesAndBusinessCombinationAcquisitionRelatedCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_4808ca70-6132-4aa3-995f-3235d0ad39c1" xlink:to="loc_mgni_RestructuringChargesAndBusinessCombinationAcquisitionRelatedCosts_a7acf227-6c5e-45bc-824d-b0371874e37e" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/MergerAcquisitionandRestructuringCostsScheduleofAccruedMergerandRestructuringCostActivityDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#MergerAcquisitionandRestructuringCostsScheduleofAccruedMergerandRestructuringCostActivityDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/MergerAcquisitionandRestructuringCostsScheduleofAccruedMergerandRestructuringCostActivityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCostAndReserveLineItems_61493d89-3a76-4127-b3cb-47dafd2710dd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestructuringCostAndReserveLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_b3fc0912-d96c-4b1e-a98f-e361f3a395d6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfRestructuringAndRelatedCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_61493d89-3a76-4127-b3cb-47dafd2710dd" xlink:to="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_b3fc0912-d96c-4b1e-a98f-e361f3a395d6" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_2f3bd4bd-2a54-4a29-a6ce-f7a33d73e6e5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_b3fc0912-d96c-4b1e-a98f-e361f3a395d6" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_2f3bd4bd-2a54-4a29-a6ce-f7a33d73e6e5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_2f3bd4bd-2a54-4a29-a6ce-f7a33d73e6e5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_2f3bd4bd-2a54-4a29-a6ce-f7a33d73e6e5" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_2f3bd4bd-2a54-4a29-a6ce-f7a33d73e6e5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_beb68e0e-0961-4ff1-9f62-ff0c376bd306" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_2f3bd4bd-2a54-4a29-a6ce-f7a33d73e6e5" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_beb68e0e-0961-4ff1-9f62-ff0c376bd306" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_SpotXAndTelariaMember_a1b419b0-40da-4918-aebe-5d1546b7188d" xlink:href="mgni-20251231.xsd#mgni_SpotXAndTelariaMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_beb68e0e-0961-4ff1-9f62-ff0c376bd306" xlink:to="loc_mgni_SpotXAndTelariaMember_a1b419b0-40da-4918-aebe-5d1546b7188d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringReserveRollForward_eeebac7a-0e4e-4255-8f08-4292db412774" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestructuringReserveRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_61493d89-3a76-4127-b3cb-47dafd2710dd" xlink:to="loc_us-gaap_RestructuringReserveRollForward_eeebac7a-0e4e-4255-8f08-4292db412774" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_RestructuringReserveAndBusinessCombinationAcquisitionRelatedCosts_de4489ad-71fb-4f65-80d8-d4bfb4c3c2ff" xlink:href="mgni-20251231.xsd#mgni_RestructuringReserveAndBusinessCombinationAcquisitionRelatedCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringReserveRollForward_eeebac7a-0e4e-4255-8f08-4292db412774" xlink:to="loc_mgni_RestructuringReserveAndBusinessCombinationAcquisitionRelatedCosts_de4489ad-71fb-4f65-80d8-d4bfb4c3c2ff" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_RestructuringChargesAndMergerRelatedCostsSettledWithoutCash_8ccb8aef-d153-469b-b23f-892d16dbb9f0" xlink:href="mgni-20251231.xsd#mgni_RestructuringChargesAndMergerRelatedCostsSettledWithoutCash"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringReserveRollForward_eeebac7a-0e4e-4255-8f08-4292db412774" xlink:to="loc_mgni_RestructuringChargesAndMergerRelatedCostsSettledWithoutCash_8ccb8aef-d153-469b-b23f-892d16dbb9f0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_RestructuringLossContracts_2a757d9f-e5e5-4bbf-adcf-68e3252f9288" xlink:href="mgni-20251231.xsd#mgni_RestructuringLossContracts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringReserveRollForward_eeebac7a-0e4e-4255-8f08-4292db412774" xlink:to="loc_mgni_RestructuringLossContracts_2a757d9f-e5e5-4bbf-adcf-68e3252f9288" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessExitCosts1_89b8f6f2-a027-44e0-8d8a-49a912c488a5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessExitCosts1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringReserveRollForward_eeebac7a-0e4e-4255-8f08-4292db412774" xlink:to="loc_us-gaap_BusinessExitCosts1_89b8f6f2-a027-44e0-8d8a-49a912c488a5" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_RestructuringImpairmentCosts_6ccf7491-1545-4420-b31a-24509a58f6cf" xlink:href="mgni-20251231.xsd#mgni_RestructuringImpairmentCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringReserveRollForward_eeebac7a-0e4e-4255-8f08-4292db412774" xlink:to="loc_mgni_RestructuringImpairmentCosts_6ccf7491-1545-4420-b31a-24509a58f6cf" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_PaymentsForRestructuringAndMergerRelatedCosts_ab686e79-8573-413d-989d-4ce21f936609" xlink:href="mgni-20251231.xsd#mgni_PaymentsForRestructuringAndMergerRelatedCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringReserveRollForward_eeebac7a-0e4e-4255-8f08-4292db412774" xlink:to="loc_mgni_PaymentsForRestructuringAndMergerRelatedCosts_ab686e79-8573-413d-989d-4ce21f936609" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_RestructuringChargesAssumedLossContractsSettledWithoutCash_5019cb9b-323b-466d-89a3-a40d28576d76" xlink:href="mgni-20251231.xsd#mgni_RestructuringChargesAssumedLossContractsSettledWithoutCash"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringReserveRollForward_eeebac7a-0e4e-4255-8f08-4292db412774" xlink:to="loc_mgni_RestructuringChargesAssumedLossContractsSettledWithoutCash_5019cb9b-323b-466d-89a3-a40d28576d76" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_RestructuringImpairmentsSettledWithoutCash_ebcc53c7-fd0a-494b-bc3e-c82715d0083f" xlink:href="mgni-20251231.xsd#mgni_RestructuringImpairmentsSettledWithoutCash"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringReserveRollForward_eeebac7a-0e4e-4255-8f08-4292db412774" xlink:to="loc_mgni_RestructuringImpairmentsSettledWithoutCash_ebcc53c7-fd0a-494b-bc3e-c82715d0083f" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_RestructuringReserveAndMergerRelatedCostsSettledWithoutCash_cfaed080-1bd5-4b34-b338-a6f23ce927b8" xlink:href="mgni-20251231.xsd#mgni_RestructuringReserveAndMergerRelatedCostsSettledWithoutCash"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringReserveRollForward_eeebac7a-0e4e-4255-8f08-4292db412774" xlink:to="loc_mgni_RestructuringReserveAndMergerRelatedCostsSettledWithoutCash_cfaed080-1bd5-4b34-b338-a6f23ce927b8" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_RestructuringReserveAndBusinessCombinationAcquisitionRelatedCosts_b387c26d-8d7e-40cb-882f-d1f72bd15f8c" xlink:href="mgni-20251231.xsd#mgni_RestructuringReserveAndBusinessCombinationAcquisitionRelatedCosts"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.magnite.com/role/SubsequentEventsDetails" xlink:type="simple" xlink:href="mgni-20251231.xsd#SubsequentEventsDetails"/>
  <link:definitionLink xlink:role="http://www.magnite.com/role/SubsequentEventsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventLineItems_c28de64b-a02c-47f2-9db0-e31be33330fd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTable_8601eb4f-5952-437f-a325-c557d7fcfb02" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SubsequentEventLineItems_c28de64b-a02c-47f2-9db0-e31be33330fd" xlink:to="loc_us-gaap_SubsequentEventTable_8601eb4f-5952-437f-a325-c557d7fcfb02" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_7ed7e4a6-78e9-4a15-9e46-4fcd4900089a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_8601eb4f-5952-437f-a325-c557d7fcfb02" xlink:to="loc_us-gaap_AwardTypeAxis_7ed7e4a6-78e9-4a15-9e46-4fcd4900089a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_7ed7e4a6-78e9-4a15-9e46-4fcd4900089a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_7ed7e4a6-78e9-4a15-9e46-4fcd4900089a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_7ed7e4a6-78e9-4a15-9e46-4fcd4900089a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_feb9608c-c25b-42ae-8435-4ce7899d48e9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_7ed7e4a6-78e9-4a15-9e46-4fcd4900089a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_feb9608c-c25b-42ae-8435-4ce7899d48e9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_0b688a2c-ba41-418f-93f5-5c9e56fc4e3a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_feb9608c-c25b-42ae-8435-4ce7899d48e9" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_0b688a2c-ba41-418f-93f5-5c9e56fc4e3a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_0d71dd0a-fc37-490a-8d59-970f6485e096" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_feb9608c-c25b-42ae-8435-4ce7899d48e9" xlink:to="loc_us-gaap_PerformanceSharesMember_0d71dd0a-fc37-490a-8d59-970f6485e096" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_019bff0e-a3dc-7abe-8ee7-728c9323461b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_feb9608c-c25b-42ae-8435-4ce7899d48e9" xlink:to="loc_us-gaap_EmployeeStockMember_019bff0e-a3dc-7abe-8ee7-728c9323461b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_2afdca9d-8afe-4ff5-bef5-a99e2b85b52e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_feb9608c-c25b-42ae-8435-4ce7899d48e9" xlink:to="loc_us-gaap_EmployeeStockOptionMember_2afdca9d-8afe-4ff5-bef5-a99e2b85b52e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingAxis_733d59b9-c0ca-4804-912c-2e67a4ffddc8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VestingAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_8601eb4f-5952-437f-a325-c557d7fcfb02" xlink:to="loc_us-gaap_VestingAxis_733d59b9-c0ca-4804-912c-2e67a4ffddc8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_733d59b9-c0ca-4804-912c-2e67a4ffddc8_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VestingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VestingAxis_733d59b9-c0ca-4804-912c-2e67a4ffddc8" xlink:to="loc_us-gaap_VestingDomain_733d59b9-c0ca-4804-912c-2e67a4ffddc8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_b5ccc4c1-a0a3-46aa-8b33-20e17fd1f2ce" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VestingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VestingAxis_733d59b9-c0ca-4804-912c-2e67a4ffddc8" xlink:to="loc_us-gaap_VestingDomain_b5ccc4c1-a0a3-46aa-8b33-20e17fd1f2ce" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_88a685b5-4043-4ebd-ae60-8b4f64a8a180" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationAwardTrancheOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_b5ccc4c1-a0a3-46aa-8b33-20e17fd1f2ce" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_88a685b5-4043-4ebd-ae60-8b4f64a8a180" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_2f25c1a6-3d31-436f-b80a-5594bb28dfbd" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_8601eb4f-5952-437f-a325-c557d7fcfb02" xlink:to="loc_srt_RangeAxis_2f25c1a6-3d31-436f-b80a-5594bb28dfbd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_2f25c1a6-3d31-436f-b80a-5594bb28dfbd_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_2f25c1a6-3d31-436f-b80a-5594bb28dfbd" xlink:to="loc_srt_RangeMember_2f25c1a6-3d31-436f-b80a-5594bb28dfbd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_0ca3a587-bd1a-4aed-badf-77c125008b8e" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_2f25c1a6-3d31-436f-b80a-5594bb28dfbd" xlink:to="loc_srt_RangeMember_0ca3a587-bd1a-4aed-badf-77c125008b8e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_b6699187-da0d-4f45-bf05-d6900ca8c20e" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_0ca3a587-bd1a-4aed-badf-77c125008b8e" xlink:to="loc_srt_MinimumMember_b6699187-da0d-4f45-bf05-d6900ca8c20e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_a85be4d4-44bc-45fc-9e1b-63504b7e2835" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_0ca3a587-bd1a-4aed-badf-77c125008b8e" xlink:to="loc_srt_MaximumMember_a85be4d4-44bc-45fc-9e1b-63504b7e2835" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ShareRepurchaseProgramAxis_019c7103-08f2-7707-bafc-7e06af1434da" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ShareRepurchaseProgramAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_8601eb4f-5952-437f-a325-c557d7fcfb02" xlink:to="loc_srt_ShareRepurchaseProgramAxis_019c7103-08f2-7707-bafc-7e06af1434da" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ShareRepurchaseProgramDomain_019c7103-08f2-7707-bafc-7e06af1434da_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ShareRepurchaseProgramDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ShareRepurchaseProgramAxis_019c7103-08f2-7707-bafc-7e06af1434da" xlink:to="loc_srt_ShareRepurchaseProgramDomain_019c7103-08f2-7707-bafc-7e06af1434da_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ShareRepurchaseProgramDomain_019c7103-43a0-70ff-a880-ceac52a46d92" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ShareRepurchaseProgramDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ShareRepurchaseProgramAxis_019c7103-08f2-7707-bafc-7e06af1434da" xlink:to="loc_srt_ShareRepurchaseProgramDomain_019c7103-43a0-70ff-a880-ceac52a46d92" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_February2026RepurchasePlanMember_019c7104-4634-79b4-add9-031464d7769c" xlink:href="mgni-20251231.xsd#mgni_February2026RepurchasePlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ShareRepurchaseProgramDomain_019c7103-43a0-70ff-a880-ceac52a46d92" xlink:to="loc_mgni_February2026RepurchasePlanMember_019c7104-4634-79b4-add9-031464d7769c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_019b7db4-2dc7-7bf9-8120-b94269ddf267" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_8601eb4f-5952-437f-a325-c557d7fcfb02" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_019b7db4-2dc7-7bf9-8120-b94269ddf267" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019b7db4-2dc7-7bf9-8120-b94269ddf267_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019b7db4-2dc7-7bf9-8120-b94269ddf267" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019b7db4-2dc7-7bf9-8120-b94269ddf267_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019b7db4-9bbe-7be6-be0c-c7857705663c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019b7db4-2dc7-7bf9-8120-b94269ddf267" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019b7db4-9bbe-7be6-be0c-c7857705663c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_019b7db5-09b3-7f7b-9748-524fc1a02f60" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_019b7db4-9bbe-7be6-be0c-c7857705663c" xlink:to="loc_us-gaap_SubsequentEventMember_019b7db5-09b3-7f7b-9748-524fc1a02f60" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_3399a753-ace2-4813-bb36-5354824d6251" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_c28de64b-a02c-47f2-9db0-e31be33330fd" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_3399a753-ace2-4813-bb36-5354824d6251" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_ce38a7da-b2cd-4de1-ac58-a258ef5ed49c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_c28de64b-a02c-47f2-9db0-e31be33330fd" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_ce38a7da-b2cd-4de1-ac58-a258ef5ed49c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_5ab6a217-af39-4c19-a695-4c0eb4e55c33" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_c28de64b-a02c-47f2-9db0-e31be33330fd" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_5ab6a217-af39-4c19-a695-4c0eb4e55c33" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StockRepurchaseProgramAuthorizedAmount1_019c75d7-137d-77b5-91b0-7e2c22497739" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_StockRepurchaseProgramAuthorizedAmount1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_c28de64b-a02c-47f2-9db0-e31be33330fd" xlink:to="loc_srt_StockRepurchaseProgramAuthorizedAmount1_019c75d7-137d-77b5-91b0-7e2c22497739" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://xbrl.sec.gov/ecd/role/IndividualsOnly" xlink:type="simple" xlink:href="https://xbrl.sec.gov/ecd/2025/ecd-2025.xsd#IndividualsOnly"/>
  <link:definitionLink xlink:role="http://xbrl.sec.gov/ecd/role/IndividualsOnly" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_ecd_AllIndividualsMember" xlink:href="https://xbrl.sec.gov/ecd/2025/ecd-2025.xsd#ecd_AllIndividualsMember"/>
    <link:loc xlink:type="locator" xlink:label="loc_mgni_DouglasKnopperMember_019b6009-31e8-7630-a700-2926ebe1cabc" xlink:href="mgni-20251231.xsd#mgni_DouglasKnopperMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecd_AllIndividualsMember" xlink:to="loc_mgni_DouglasKnopperMember_019b6009-31e8-7630-a700-2926ebe1cabc" xlink:type="arc" order="2"/>
  </link:definitionLink>
</link:linkbase>
