diff --git a/.github/workflows/end-to-end-tests.yml b/.github/workflows/end-to-end-tests.yml
index 3c37381a52..aa83092d84 100644
--- a/.github/workflows/end-to-end-tests.yml
+++ b/.github/workflows/end-to-end-tests.yml
@@ -16,6 +16,7 @@ jobs:
   # Runs the end-to-end test suite.
   #
   # Performs the following steps:
+  # - Cancels all previous workflow runs for pull requests that have not completed.
   # - Set environment variables.
   # - Checks out the repository.
   # - Logs debug information about the runner container.
@@ -34,6 +35,12 @@ jobs:
     name: E2E Tests
     runs-on: ubuntu-latest
     steps:
+      - name: Cancel previous runs of this workflow (pull requests only)
+        if: ${{ github.event_name == 'pull_request' }}
+        uses: styfle/cancel-workflow-action@0.5.0
+        with:
+          access_token: ${{ github.token }}
+
       - name: Configure environment variables
         run: |
           echo "PHP_FPM_UID=$(id -u)" >> $GITHUB_ENV
diff --git a/.github/workflows/javascript-tests.yml b/.github/workflows/javascript-tests.yml
index 40b36dfd9a..aae2476e57 100644
--- a/.github/workflows/javascript-tests.yml
+++ b/.github/workflows/javascript-tests.yml
@@ -11,6 +11,7 @@ jobs:
   # Runs the QUnit tests for WordPress.
   #
   # Performs the following steps:
+  # - Cancels all previous workflow runs for pull requests that have not completed.
   # - Checks out the repository.
   # - Logs debug information about the runner container.
   # - Installs NodeJS 12 (todo: install the version of NPM specified in the `.nvmrc` file to support older branches)
@@ -23,6 +24,12 @@ jobs:
     name: QUnit Tests
     runs-on: ubuntu-latest
     steps:
+      - name: Cancel previous runs of this workflow (pull requests only)
+        if: ${{ github.event_name == 'pull_request' }}
+        uses: styfle/cancel-workflow-action@0.5.0
+        with:
+          access_token: ${{ github.token }}
+
       - name: Checkout repository
         uses: actions/checkout@v2
 
diff --git a/.github/workflows/phpunit-tests.yml b/.github/workflows/phpunit-tests.yml
index b024a1979b..16ef65deae 100644
--- a/.github/workflows/phpunit-tests.yml
+++ b/.github/workflows/phpunit-tests.yml
@@ -22,6 +22,7 @@ jobs:
   # Sets up WordPress for testing or development use.
   #
   # Performs the following steps:
+  # - Cancels all previous workflow runs for pull requests that have not completed.
   # - Checks out the repository.
   # - Checks out the WordPress Importer plugin (needed for the Core PHPUnit tests).
   # - Logs debug information about the runner container.
@@ -36,6 +37,12 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
+      - name: Cancel previous runs of this workflow (pull requests only)
+        if: ${{ github.event_name == 'pull_request' }}
+        uses: styfle/cancel-workflow-action@0.5.0
+        with:
+          access_token: ${{ github.token }}
+
       - name: Checkout repository
         uses: actions/checkout@v2
 
diff --git a/.github/workflows/verify-npm-on-windows.yml b/.github/workflows/verify-npm-on-windows.yml
index 1480e301c9..e01915a61f 100644
--- a/.github/workflows/verify-npm-on-windows.yml
+++ b/.github/workflows/verify-npm-on-windows.yml
@@ -14,6 +14,7 @@ jobs:
   # Verifies that installing NPM dependencies and building WordPress works on Windows.
   #
   # Performs the following steps:
+  # - Cancels all previous workflow runs for pull requests that have not completed.
   # - Checks out the repository.
   # - Logs debug information about the runner container.
   # - Installs NodeJS 12 (todo: install the version of NPM specified in the `.nvmrc` file to support older branches)
@@ -24,6 +25,12 @@ jobs:
     name: Tests NPM on Windows
     runs-on: windows-latest
     steps:
+      - name: Cancel previous runs of this workflow (pull requests only)
+        if: ${{ github.event_name == 'pull_request' }}
+        uses: styfle/cancel-workflow-action@0.5.0
+        with:
+          access_token: ${{ github.token }}
+
       - name: Checkout repository
         uses: actions/checkout@v2