diff --git a/.github/workflows/mirror-main-to-master.yml b/.github/workflows/mirror-main-to-master.yml
new file mode 100644
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+++ b/.github/workflows/mirror-main-to-master.yml
@@ -0,0 +1,18 @@
+# The default branch is "main", but older checkouts may still use "master".
+# This workflow keeps "master" up to date with "main".
+name: Mirror main to master
+
+on:
+  push:
+    branches: [ main ]
+
+jobs:
+  mirror-main-to-master:
+    runs-on: ubuntu-latest
+    if: github.repository == 'github/vscode-codeql-starter'
+
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Push main to master
+        run: git push origin main:master