diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7a6ce07e7b5a..30fdd030c1c4 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -16,6 +16,7 @@
 - `[jest-mock]` Fix inheritance of static properties and methods in mocks ([#7003](https://github.com/facebook/jest/pull/7003))
 - `[jest-mock]` Fix mocking objects without `Object.prototype` in their prototype chain ([#7003](https://github.com/facebook/jest/pull/7003))
 - `[jest-cli]` Update jest-cli to show git ref in message when using `changedSince` ([#7028](https://github.com/facebook/jest/pull/7028))
+- `[jest-runtime]` Check `globalMock._isMockFunction` is true rather than truthy ([#7017](https://github.com/facebook/jest/pull/7017))
 
 ### Chore & Maintenance
 
diff --git a/packages/jest-mock/src/index.js b/packages/jest-mock/src/index.js
index 39bb7a8a8d02..70e8260a0df3 100644
--- a/packages/jest-mock/src/index.js
+++ b/packages/jest-mock/src/index.js
@@ -687,7 +687,7 @@ class ModuleMockerClass {
       return metadata;
     } else if (type === 'function') {
       metadata.name = component.name;
-      if (component._isMockFunction) {
+      if (component._isMockFunction === true) {
         metadata.mockImpl = component.getMockImplementation();
       }
     }
@@ -702,7 +702,7 @@ class ModuleMockerClass {
         this._getSlots(component).forEach(slot => {
           if (
             type === 'function' &&
-            component._isMockFunction &&
+            component._isMockFunction === true &&
             slot.match(/^mock/)
           ) {
             return;
@@ -727,7 +727,7 @@ class ModuleMockerClass {
   }
 
   isMockFunction(fn: any): boolean {
-    return !!(fn && fn._isMockFunction);
+    return !!(fn && fn._isMockFunction === true);
   }
 
   fn(implementation?: any): any {
diff --git a/packages/jest-runtime/src/index.js b/packages/jest-runtime/src/index.js
index d6612eb42221..507da20aa247 100644
--- a/packages/jest-runtime/src/index.js
+++ b/packages/jest-runtime/src/index.js
@@ -430,7 +430,7 @@ class Runtime {
           (typeof globalMock === 'object' && globalMock !== null) ||
           typeof globalMock === 'function'
         ) {
-          globalMock._isMockFunction && globalMock.mockClear();
+          globalMock._isMockFunction === true && globalMock.mockClear();
         }
       });