diff --git a/httpx/_transports/mock.py b/httpx/_transports/mock.py
index 8c418f59e0..f0b8aa0007 100644
--- a/httpx/_transports/mock.py
+++ b/httpx/_transports/mock.py
@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import datetime
 import typing
 
 from .._models import Request, Response
@@ -24,6 +25,8 @@ def handle_request(
         response = self.handler(request)
         if not isinstance(response, Response):  # pragma: no cover
             raise TypeError("Cannot use an async handler in a sync Client")
+        if not hasattr(response, "_elapsed"):
+            response.elapsed = datetime.timedelta(0)
         return response
 
     async def handle_async_request(
@@ -40,4 +43,6 @@ async def handle_async_request(
         if not isinstance(response, Response):
             response = await response
 
+        if not hasattr(response, "_elapsed"):
+            response.elapsed = datetime.timedelta(0)
         return response