diff --git a/packages/test/src/test-mockactivityenv.ts b/packages/test/src/test-mockactivityenv.ts
index 6811cf06b..cfdc2eb35 100644
--- a/packages/test/src/test-mockactivityenv.ts
+++ b/packages/test/src/test-mockactivityenv.ts
@@ -47,3 +47,13 @@ test('MockActivityEnvironment injects provided info', async (t) => {
   }, 1);
   t.is(res, 4);
 });
+
+test('MockActivityEnvironment return type is correctly inferred', async (t) => {
+  async function foo(): Promise<string> {
+    return 'foo';
+  }
+  const result = await new MockActivityEnvironment().run(foo);
+  // There should be no compile time error on this line
+  result.startsWith('foo');
+  t.pass();
+});
diff --git a/packages/testing/src/mocking-activity-environment.ts b/packages/testing/src/mocking-activity-environment.ts
index b9f9c05b0..b6a87cfcd 100644
--- a/packages/testing/src/mocking-activity-environment.ts
+++ b/packages/testing/src/mocking-activity-environment.ts
@@ -58,8 +58,8 @@ export class MockActivityEnvironment extends events.EventEmitter {
   /**
    * Run a function in Activity Context
    */
-  public async run<P extends any[], R, F extends ActivityFunction<P, R>>(fn: F, ...args: P): Promise<R> {
-    return this.activity.runNoEncoding(fn as ActivityFunction<any, any>, { args, headers: {} }) as Promise<R>;
+  public run<F extends ActivityFunction>(fn: F, ...args: Parameters<F>): ReturnType<F> {
+    return this.activity.runNoEncoding(fn, { args, headers: {} }) as ReturnType<F>;
   }
 }
 
diff --git a/packages/worker/src/activity.ts b/packages/worker/src/activity.ts
index 7614017d0..0c2103d37 100644
--- a/packages/worker/src/activity.ts
+++ b/packages/worker/src/activity.ts
@@ -193,9 +193,9 @@ export class Activity {
     });
   }
 
-  public runNoEncoding(fn: ActivityFunction<any[], any>, input: ActivityExecuteInput): Promise<unknown> {
+  public runNoEncoding<F extends ActivityFunction>(fn: F, input: ActivityExecuteInput): ReturnType<F> {
     if (this.fn !== undefined) throw new IllegalStateError('Activity function is defined');
-    return asyncLocalStorage.run(this.context, () => this.execute(fn, input));
+    return asyncLocalStorage.run(this.context, () => this.execute(fn, input)) as ReturnType<F>;
   }
 }