diff --git a/.changeset/fluffy-badgers-rush.md b/.changeset/fluffy-badgers-rush.md
new file mode 100644
index 00000000000..62f55a2b0f0
--- /dev/null
+++ b/.changeset/fluffy-badgers-rush.md
@@ -0,0 +1,5 @@
+---
+"@apollo/client": patch
+---
+
+MockLink: add query default variables if not specified in mock request
diff --git a/src/testing/core/mocking/__tests__/mockLink.ts b/src/testing/core/mocking/__tests__/mockLink.ts
index 4207a42503a..fe618ae4f48 100644
--- a/src/testing/core/mocking/__tests__/mockLink.ts
+++ b/src/testing/core/mocking/__tests__/mockLink.ts
@@ -1,7 +1,7 @@
 import gql from "graphql-tag";
 import { MockLink, MockedResponse } from "../mockLink";
 import { execute } from "../../../../link/core/execute";
-import { ObservableStream } from "../../../internal";
+import { ObservableStream, enableFakeTimers } from "../../../internal";
 
 describe("MockedResponse.newData", () => {
   const setup = () => {
@@ -73,9 +73,6 @@ We've chosen this value as the MAXIMUM_DELAY since values that don't fit into a
 const MAXIMUM_DELAY = 0x7f_ff_ff_ff;
 
 describe("mockLink", () => {
-  beforeAll(() => jest.useFakeTimers());
-  afterAll(() => jest.useRealTimers());
-
   const query = gql`
     query A {
       a
@@ -83,6 +80,8 @@ describe("mockLink", () => {
   `;
 
   it("should not require a result or error when delay equals Infinity", async () => {
+    using _fakeTimers = enableFakeTimers();
+
     const mockLink = new MockLink([
       {
         request: {
@@ -104,6 +103,8 @@ describe("mockLink", () => {
   });
 
   it("should require result or error when delay is just large", (done) => {
+    using _fakeTimers = enableFakeTimers();
+
     const mockLink = new MockLink([
       {
         request: {
@@ -126,6 +127,81 @@ describe("mockLink", () => {
 
     jest.advanceTimersByTime(MAXIMUM_DELAY);
   });
+
+  it("should fill in default variables if they are missing in mocked requests", async () => {
+    const query = gql`
+      query GetTodo($done: Boolean = true, $user: String!) {
+        todo(user: $user, done: $done) {
+          id
+        }
+      }
+    `;
+    const mocks = [
+      {
+        // default should get filled in here
+        request: { query, variables: { user: "Tim" } },
+        result: {
+          data: { todo: { id: 1 } },
+        },
+      },
+      {
+        // we provide our own `done`, so it should not get filled in
+        request: { query, variables: { user: "Tim", done: false } },
+        result: {
+          data: { todo: { id: 2 } },
+        },
+      },
+      {
+        // one more that has a different user variable and should never match
+        request: { query, variables: { user: "Tom" } },
+        result: {
+          data: { todo: { id: 2 } },
+        },
+      },
+    ];
+
+    // Apollo Client will always fill in default values for missing variables
+    // in the operation before calling the Link, so we have to do the same here
+    // when we call `execute`
+    const defaults = { done: true };
+    const link = new MockLink(mocks, false, { showWarnings: false });
+    {
+      // Non-optional variable is missing, should not match.
+      const stream = new ObservableStream(
+        execute(link, { query, variables: { ...defaults } })
+      );
+      await stream.takeError();
+    }
+    {
+      // Execute called incorrectly without a default variable filled in.
+      // This will never happen in Apollo Client since AC always fills these
+      // before calling `execute`, so it's okay if it results in a "no match"
+      // scenario here.
+      const stream = new ObservableStream(
+        execute(link, { query, variables: { user: "Tim" } })
+      );
+      await stream.takeError();
+    }
+    {
+      // Expect default value to be filled in the mock request.
+      const stream = new ObservableStream(
+        execute(link, { query, variables: { ...defaults, user: "Tim" } })
+      );
+      const result = await stream.takeNext();
+      expect(result).toEqual({ data: { todo: { id: 1 } } });
+    }
+    {
+      // Test that defaults don't overwrite explicitly different values in a mock request.
+      const stream = new ObservableStream(
+        execute(link, {
+          query,
+          variables: { ...defaults, user: "Tim", done: false },
+        })
+      );
+      const result = await stream.takeNext();
+      expect(result).toEqual({ data: { todo: { id: 2 } } });
+    }
+  });
 });
 
 test("removes @nonreactive directives from fields", async () => {
diff --git a/src/testing/core/mocking/mockLink.ts b/src/testing/core/mocking/mockLink.ts
index f38474e1c20..646567b2da9 100644
--- a/src/testing/core/mocking/mockLink.ts
+++ b/src/testing/core/mocking/mockLink.ts
@@ -16,6 +16,8 @@ import {
   cloneDeep,
   stringifyForDisplay,
   print,
+  getOperationDefinition,
+  getDefaultValues,
   removeDirectivesFromDocument,
   checkDocument,
 } from "../../../utilities/index.js";
@@ -233,17 +235,21 @@ ${unmatchedVars.map((d) => `  ${stringifyForDisplay(d)}`).join("\n")}
   }
 
   private normalizeVariableMatching(mockedResponse: MockedResponse) {
-    const variables = mockedResponse.request.variables;
-    if (mockedResponse.variableMatcher && variables) {
+    const request = mockedResponse.request;
+    if (mockedResponse.variableMatcher && request.variables) {
       throw new Error(
         "Mocked response should contain either variableMatcher or request.variables"
       );
     }
 
     if (!mockedResponse.variableMatcher) {
+      request.variables = {
+        ...getDefaultValues(getOperationDefinition(request.query)),
+        ...request.variables,
+      };
       mockedResponse.variableMatcher = (vars) => {
         const requestVariables = vars || {};
-        const mockedResponseVariables = variables || {};
+        const mockedResponseVariables = request.variables || {};
         return equal(requestVariables, mockedResponseVariables);
       };
     }
diff --git a/src/testing/internal/disposables/enableFakeTimers.ts b/src/testing/internal/disposables/enableFakeTimers.ts
new file mode 100644
index 00000000000..6be85d24730
--- /dev/null
+++ b/src/testing/internal/disposables/enableFakeTimers.ts
@@ -0,0 +1,26 @@
+import { withCleanup } from "./withCleanup.js";
+
+declare global {
+  interface DateConstructor {
+    /* Jest uses @sinonjs/fake-timers, that add this flag */
+    isFake: boolean;
+  }
+}
+
+export function enableFakeTimers(
+  config?: FakeTimersConfig | LegacyFakeTimersConfig
+) {
+  if (global.Date.isFake === true) {
+    // Nothing to do here, fake timers have already been set up.
+    // That also means we don't want to clean that up later.
+    return withCleanup({}, () => {});
+  }
+
+  jest.useFakeTimers(config);
+  return withCleanup({}, () => {
+    if (global.Date.isFake === true) {
+      jest.runOnlyPendingTimers();
+      jest.useRealTimers();
+    }
+  });
+}
diff --git a/src/testing/internal/disposables/index.ts b/src/testing/internal/disposables/index.ts
index 9895d129589..9d61c88fd90 100644
--- a/src/testing/internal/disposables/index.ts
+++ b/src/testing/internal/disposables/index.ts
@@ -1,3 +1,4 @@
 export { disableActWarnings } from "./disableActWarnings.js";
 export { spyOnConsole } from "./spyOnConsole.js";
 export { withCleanup } from "./withCleanup.js";
+export { enableFakeTimers } from "./enableFakeTimers.js";