diff --git a/src/when.js b/src/when.js
index 39f1742..230ba48 100644
--- a/src/when.js
+++ b/src/when.js
@@ -88,8 +88,9 @@ class WhenMock {
             return mockImplementation(...args)
           }
         }
-
-        return defaultImplementation ? defaultImplementation(...args) : undefined
+        return defaultImplementation ? defaultImplementation(...args)
+          : (typeof fn.__whenMock__._origMock === 'function'
+            ? fn.__whenMock__._origMock(...args) : undefined)
       })
 
       return {
diff --git a/src/when.test.js b/src/when.test.js
index 506be97..0b81bb1 100644
--- a/src/when.test.js
+++ b/src/when.test.js
@@ -819,5 +819,30 @@ describe('When', () => {
       const returnValue = theInstance.theMethod(1)
       expect(returnValue).toBe('mock')
     })
+
+    it('keeps default function implementation when not matched', () => {
+      class TheClass {
+        fn () {
+          return 'real'
+        }
+      }
+      const instance = new TheClass()
+      const spy = jest.spyOn(instance, 'fn')
+      when(spy)
+        .calledWith(1)
+        .mockReturnValue('mock')
+      expect(instance.fn(2)).toBe('real')
+    })
+
+    it('keeps default mock implementation when not matched', () => {
+      const fn = jest.fn(() => {
+        return 'real'
+      })
+      when(fn)
+        .calledWith(1)
+        .mockReturnValue('mock')
+      expect(fn(1)).toBe('mock')
+      expect(fn(2)).toBe('real')
+    })
   })
 })