diff --git a/dev-infra/ng-dev.js b/dev-infra/ng-dev.js
index 39e9f71014..04d29d9501 100755
--- a/dev-infra/ng-dev.js
+++ b/dev-infra/ng-dev.js
@@ -3900,35 +3900,52 @@ var AutosquashMergeStrategy = /** @class */ (function (_super) {
      */
     AutosquashMergeStrategy.prototype.merge = function (pullRequest) {
         return tslib.__awaiter(this, void 0, void 0, function () {
-            var prNumber, targetBranches, requiredBaseSha, needsCommitMessageFixup, baseSha, revisionRange, branchOrRevisionBeforeRebase, rebaseEnv, failedBranches;
+            var prNumber, targetBranches, requiredBaseSha, needsCommitMessageFixup, githubTargetBranch, baseSha, revisionRange, branchOrRevisionBeforeRebase, rebaseEnv, failedBranches, localBranch, sha;
             return tslib.__generator(this, function (_a) {
-                prNumber = pullRequest.prNumber, targetBranches = pullRequest.targetBranches, requiredBaseSha = pullRequest.requiredBaseSha, needsCommitMessageFixup = pullRequest.needsCommitMessageFixup;
-                // In case a required base is specified for this pull request, check if the pull
-                // request contains the given commit. If not, return a pull request failure. This
-                // check is useful for enforcing that PRs are rebased on top of a given commit. e.g.
-                // a commit that changes the codeowner ship validation. PRs which are not rebased
-                // could bypass new codeowner ship rules.
-                if (requiredBaseSha && !this.git.hasCommit(TEMP_PR_HEAD_BRANCH, requiredBaseSha)) {
-                    return [2 /*return*/, PullRequestFailure.unsatisfiedBaseSha()];
+                switch (_a.label) {
+                    case 0:
+                        prNumber = pullRequest.prNumber, targetBranches = pullRequest.targetBranches, requiredBaseSha = pullRequest.requiredBaseSha, needsCommitMessageFixup = pullRequest.needsCommitMessageFixup, githubTargetBranch = pullRequest.githubTargetBranch;
+                        // In case a required base is specified for this pull request, check if the pull
+                        // request contains the given commit. If not, return a pull request failure. This
+                        // check is useful for enforcing that PRs are rebased on top of a given commit. e.g.
+                        // a commit that changes the codeowner ship validation. PRs which are not rebased
+                        // could bypass new codeowner ship rules.
+                        if (requiredBaseSha && !this.git.hasCommit(TEMP_PR_HEAD_BRANCH, requiredBaseSha)) {
+                            return [2 /*return*/, PullRequestFailure.unsatisfiedBaseSha()];
+                        }
+                        baseSha = this.git.run(['rev-parse', this.getPullRequestBaseRevision(pullRequest)]).stdout.trim();
+                        revisionRange = baseSha + ".." + TEMP_PR_HEAD_BRANCH;
+                        branchOrRevisionBeforeRebase = this.git.getCurrentBranchOrRevision();
+                        rebaseEnv = needsCommitMessageFixup ? undefined : tslib.__assign(tslib.__assign({}, process.env), { GIT_SEQUENCE_EDITOR: 'true' });
+                        this.git.run(['rebase', '--interactive', '--autosquash', baseSha, TEMP_PR_HEAD_BRANCH], { stdio: 'inherit', env: rebaseEnv });
+                        // Update pull requests commits to reference the pull request. This matches what
+                        // Github does when pull requests are merged through the Web UI. The motivation is
+                        // that it should be easy to determine which pull request contained a given commit.
+                        // Note: The filter-branch command relies on the working tree, so we want to make sure
+                        // that we are on the initial branch or revision where the merge script has been invoked.
+                        this.git.run(['checkout', '-f', branchOrRevisionBeforeRebase]);
+                        this.git.run(['filter-branch', '-f', '--msg-filter', MSG_FILTER_SCRIPT + " " + prNumber, revisionRange]);
+                        failedBranches = this.cherryPickIntoTargetBranches(revisionRange, targetBranches);
+                        if (failedBranches.length) {
+                            return [2 /*return*/, PullRequestFailure.mergeConflicts(failedBranches)];
+                        }
+                        this.pushTargetBranchesUpstream(targetBranches);
+                        if (!(githubTargetBranch !== 'master')) return [3 /*break*/, 3];
+                        localBranch = this.getLocalTargetBranchName(githubTargetBranch);
+                        sha = this.git.run(['rev-parse', localBranch]).stdout.trim();
+                        // Create a comment saying the PR was closed by the SHA.
+                        return [4 /*yield*/, this.git.github.issues.createComment(tslib.__assign(tslib.__assign({}, this.git.remoteParams), { issue_number: pullRequest.prNumber, body: "Closed by commit " + sha }))];
+                    case 1:
+                        // Create a comment saying the PR was closed by the SHA.
+                        _a.sent();
+                        // Actually close the PR.
+                        return [4 /*yield*/, this.git.github.pulls.update(tslib.__assign(tslib.__assign({}, this.git.remoteParams), { pull_number: pullRequest.prNumber, state: 'closed' }))];
+                    case 2:
+                        // Actually close the PR.
+                        _a.sent();
+                        _a.label = 3;
+                    case 3: return [2 /*return*/, null];
                 }
-                baseSha = this.git.run(['rev-parse', this.getPullRequestBaseRevision(pullRequest)]).stdout.trim();
-                revisionRange = baseSha + ".." + TEMP_PR_HEAD_BRANCH;
-                branchOrRevisionBeforeRebase = this.git.getCurrentBranchOrRevision();
-                rebaseEnv = needsCommitMessageFixup ? undefined : tslib.__assign(tslib.__assign({}, process.env), { GIT_SEQUENCE_EDITOR: 'true' });
-                this.git.run(['rebase', '--interactive', '--autosquash', baseSha, TEMP_PR_HEAD_BRANCH], { stdio: 'inherit', env: rebaseEnv });
-                // Update pull requests commits to reference the pull request. This matches what
-                // Github does when pull requests are merged through the Web UI. The motivation is
-                // that it should be easy to determine which pull request contained a given commit.
-                // Note: The filter-branch command relies on the working tree, so we want to make sure
-                // that we are on the initial branch or revision where the merge script has been invoked.
-                this.git.run(['checkout', '-f', branchOrRevisionBeforeRebase]);
-                this.git.run(['filter-branch', '-f', '--msg-filter', MSG_FILTER_SCRIPT + " " + prNumber, revisionRange]);
-                failedBranches = this.cherryPickIntoTargetBranches(revisionRange, targetBranches);
-                if (failedBranches.length) {
-                    return [2 /*return*/, PullRequestFailure.mergeConflicts(failedBranches)];
-                }
-                this.pushTargetBranchesUpstream(targetBranches);
-                return [2 /*return*/, null];
             });
         });
     };
diff --git a/dev-infra/pr/merge/strategies/autosquash-merge.ts b/dev-infra/pr/merge/strategies/autosquash-merge.ts
index e304ed98bd..f4d72c4680 100644
--- a/dev-infra/pr/merge/strategies/autosquash-merge.ts
+++ b/dev-infra/pr/merge/strategies/autosquash-merge.ts
@@ -32,7 +32,8 @@ export class AutosquashMergeStrategy extends MergeStrategy {
    * @returns A pull request failure or null in case of success.
    */
   async merge(pullRequest: PullRequest): Promise<PullRequestFailure|null> {
-    const {prNumber, targetBranches, requiredBaseSha, needsCommitMessageFixup} = pullRequest;
+    const {prNumber, targetBranches, requiredBaseSha, needsCommitMessageFixup, githubTargetBranch} =
+        pullRequest;
     // In case a required base is specified for this pull request, check if the pull
     // request contains the given commit. If not, return a pull request failure. This
     // check is useful for enforcing that PRs are rebased on top of a given commit. e.g.
@@ -83,6 +84,30 @@ export class AutosquashMergeStrategy extends MergeStrategy {
     }
 
     this.pushTargetBranchesUpstream(targetBranches);
+
+    // For PRs which do not target the `master` branch on Github, Github does not automatically
+    // close the PR when its commit is pushed into the repository.  To ensure these PRs are
+    // correctly marked as closed, we must detect this situation and close the PR via the API after
+    // the upstream pushes are completed.
+    if (githubTargetBranch !== 'master') {
+      /** The local branch name of the github targeted branch. */
+      const localBranch = this.getLocalTargetBranchName(githubTargetBranch);
+      /** The SHA of the commit pushed to github which represents closing the PR. */
+      const sha = this.git.run(['rev-parse', localBranch]).stdout.trim();
+      // Create a comment saying the PR was closed by the SHA.
+      await this.git.github.issues.createComment({
+        ...this.git.remoteParams,
+        issue_number: pullRequest.prNumber,
+        body: `Closed by commit ${sha}`
+      });
+      // Actually close the PR.
+      await this.git.github.pulls.update({
+        ...this.git.remoteParams,
+        pull_number: pullRequest.prNumber,
+        state: 'closed',
+      });
+    }
+
     return null;
   }
 }