diff --git a/.github/workflows/build-emhermesc.yml b/.github/workflows/build-emhermesc.yml
new file mode 100644
index 0000000000..deab163333
--- /dev/null
+++ b/.github/workflows/build-emhermesc.yml
@@ -0,0 +1,98 @@
+name: Build and commit/verify emhermesc.js
+
+on:
+  push:
+    branches:
+      - "main"
+    paths:
+      - "packages/metro-hermes-compiler/**"
+      - ".github/workflows/build-emhermesc.yml"
+  pull_request:
+    paths:
+      - "packages/metro-hermes-compiler/**"
+      - ".github/workflows/build-emhermesc.yml"
+
+jobs:
+  build-emhermesc-js:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout code (PR)
+        uses: actions/checkout@v2.0.0
+        # https://github.com/stefanzweifel/git-auto-commit-action#checkout-the-correct-branch
+        if: github.event_name == 'pull_request'
+        with:
+          ref: ${{ github.head_ref }}
+
+      - name: Checkout code (Push)
+        uses: actions/checkout@v2.0.0
+        if: github.event_name == 'push'
+
+      - name: Determine target revision from packages/metro-hermes-compiler/hermes-github-ref
+        run: echo "HERMES_GITHUB_REF=`cat packages/metro-hermes-compiler/hermes-github-ref`" >> $GITHUB_ENV
+
+      - name: Download hermes at ${{ env.HERMES_GITHUB_REF }}
+        run: |
+          mkdir $RUNNER_TEMP/hermes && curl \
+            -L https://api.github.com/repos/facebook/hermes/tarball/${{ env.HERMES_GITHUB_REF }} \
+            | tar xzf - -C $RUNNER_TEMP/hermes \
+            && echo "HERMES_CHECKOUT=`ls -d $RUNNER_TEMP/hermes/*`" >> $GITHUB_ENV
+
+      - name: Setup Ninja
+        uses: seanmiddleditch/gha-setup-ninja@856654e80dd21909aec244cd01bb96e5c841d64f
+        with:
+          destination: ${{ runner.temp }}/ninja
+
+      - name: Configure build_host_hermes
+        run: python3 $HERMES_CHECKOUT/utils/build/configure.py $RUNNER_TEMP/build_host_hermes
+
+      - name: Build build_host_hermes
+        run: cmake --build $RUNNER_TEMP/build_host_hermes --target hermesc
+
+      - name: Setup Emscripten SDK
+        uses: mymindstorm/setup-emsdk@v11
+
+      - name: Configure Emscripten hermes build
+        run: |
+          cmake $HERMES_CHECKOUT \
+            -B $RUNNER_TEMP/embuild \
+            -DCMAKE_TOOLCHAIN_FILE=$EMSDK/upstream/emscripten/cmake/Modules/Platform/Emscripten.cmake \
+            -DCMAKE_BUILD_TYPE=Release \
+            -DCMAKE_EXE_LINKER_FLAGS=" \
+              -s BINARYEN_ASYNC_COMPILATION=0 \
+              -s ENVIRONMENT=node \
+              -s EXPORT_NAME=createHermesc \
+              -s EXTRA_EXPORTED_RUNTIME_METHODS=[cwrap,ccall] \
+              -s MODULARIZE=1 \
+              -s NODERAWFS=1 \
+              -s WASM=1 \
+              -s ALLOW_MEMORY_GROWTH=1 \
+              -s NODEJS_CATCH_EXIT=0 \
+              -s NODEJS_CATCH_REJECTION=0 \
+              -s WASM_ASYNC_COMPILATION=0 \
+              -s SINGLE_FILE=1 \
+              " \
+            -DIMPORT_HERMESC:PATH="$RUNNER_TEMP/build_host_hermes/ImportHermesc.cmake"
+
+      - name: Build emhermesc
+        run: cmake --build $RUNNER_TEMP/embuild --target emhermesc
+
+      - name: Concatenate with emhermesc.js.header and move to packages/metro-hermes-compiler/src/emhermesc.js
+        run: |
+          cat packages/metro-hermes-compiler/src/emhermesc.js.header $RUNNER_TEMP/embuild/bin/emhermesc.js \
+          > packages/metro-hermes-compiler/src/emhermesc.js
+
+      - name: Upload emhermesc.js as a CI artifact
+        uses: actions/upload-artifact@v3
+        with:
+          name: emhermesc.js
+          path: packages/metro-hermes-compiler/src/emhermesc.js
+
+      - name: Commit updated emhermesc.js (PR only)
+        uses: stefanzweifel/git-auto-commit-action@d0487b9fa3a792b5e90562c27541eedecc2548b4
+        if: github.event_name == 'pull_request'
+        with:
+          commit_message: "Auto: Build emhermesc.js from hermes/${{ env.HERMES_REVISION }}"
+
+      - name: Fail if emhermesc.js is out of sync with hermes-revision
+        run: if [ -n "$(git status --porcelain)" ]; then exit 1; fi
diff --git a/packages/metro-hermes-compiler/README.md b/packages/metro-hermes-compiler/README.md
index 57f6368242..9775ae466d 100644
--- a/packages/metro-hermes-compiler/README.md
+++ b/packages/metro-hermes-compiler/README.md
@@ -4,15 +4,14 @@ This experimental module provides a high-level API to work with the Hermes bytec
 
 ## How to build HBC
 
-A pre-configured emscripten environment can be used through this [Docker image](https://hub.docker.com/r/trzeci/emscripten/). Docker can be installed via its [desktop app](https://docs.docker.com/docker-for-mac/). Make sure to increase resource limits (16G RAM, as much CPU as possible).
+[`./src/emhermesc.js`](https://github.com/rh389/metro/blob/main/packages/metro-hermes-compiler/src/emhermesc.js) is built by a [GitHub action on the Metro repository](https://github.com/rh389/metro/blob/main/.github/workflows/build-emhermesc.yml). It works by:
 
-```
-cd path/to/hermes/checkout
-docker run -i -t --rm -v `pwd`:`pwd` trzeci/emscripten bash
-apt-get update -y && apt-get install -y icu-devtools
-cd path/to/hermes/checkout
-cmake . -DCMAKE_TOOLCHAIN_FILE=/emsdk_portable/emscripten/sdk/cmake/Modules/Platform/Emscripten.cmake -DCMAKE_BUILD_TYPE=Release
-make -j emhermesc
-```
+1. Checking out the [`facebook/hermes`](https://github.com/facebook/hermes) repo at a particular GitHub `ref` (commit or tag) specified by [`./hermes-github-ref`](https://github.com/rh389/metro/blob/main/packages/metro-hermes-compiler/hermes-github-ref)
+2. Following the two stage process outlined at [`facebook/hermes/doc/Emscripten.md`](https://github.com/facebook/hermes/blob/17d632d0802a0a3fb97a962ede6a6291e5029c84/doc/Emscripten.md) to build with Emscripten and CMake, and
+3. Prepending our custom header from [`./src/emhermesc.js.header`](https://github.com/rh389/metro/blob/main/packages/metro-hermes-compiler/src/emhermesc.js.header).
 
-After the build process finishes the Hermes Bytecode Compiler JavaScript file will be available in the `bin` folder.
+To update the build, open a GitHub PR (or export one from Phabricator) that changes the contents of `hermes-github-ref` to the new target `ref`. The GH action should run on the PR and then push a new commit to the PR branch (if it has access) with the updated `emhermesc.js` - that's it!
+
+The PR can then be (re-)imported and pushed. The action will run again on a push to `main` to verify that `hermes-github-ref` and `emhermesc.js` are in sync.
+
+**If** the action is able to build `emhermesc.js` but doesn't have permission to push to the PR branch, or fails validation on `main`, you can still use the build output and update it manually - you'll find it uploaded as an artifact on the action instance.
\ No newline at end of file
diff --git a/packages/metro-hermes-compiler/hermes-github-ref b/packages/metro-hermes-compiler/hermes-github-ref
new file mode 100644
index 0000000000..4d7676e774
--- /dev/null
+++ b/packages/metro-hermes-compiler/hermes-github-ref
@@ -0,0 +1 @@
+45e487103aa121bdaab14653abe29d720702d930
\ No newline at end of file
diff --git a/packages/metro-hermes-compiler/src/emhermesc.js.header b/packages/metro-hermes-compiler/src/emhermesc.js.header
new file mode 100644
index 0000000000..6b1a60620d
--- /dev/null
+++ b/packages/metro-hermes-compiler/src/emhermesc.js.header
@@ -0,0 +1,10 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @generated
+ */
+
+/* eslint-disable */