diff --git a/google/cloud/storage/google_cloud_cpp_storage.cmake b/google/cloud/storage/google_cloud_cpp_storage.cmake
index 5e53e972a990..d475554d4573 100644
--- a/google/cloud/storage/google_cloud_cpp_storage.cmake
+++ b/google/cloud/storage/google_cloud_cpp_storage.cmake
@@ -406,7 +406,6 @@ if (BUILD_TESTING)
         testing/client_unit_test.cc
         testing/client_unit_test.h
         testing/constants.h
-        testing/mock_client.cc
         testing/mock_client.h
         testing/mock_http_request.cc
         testing/mock_http_request.h
diff --git a/google/cloud/storage/storage_client_testing.bzl b/google/cloud/storage/storage_client_testing.bzl
index 5727e9f9a90f..e060351a27fa 100644
--- a/google/cloud/storage/storage_client_testing.bzl
+++ b/google/cloud/storage/storage_client_testing.bzl
@@ -35,7 +35,6 @@ storage_client_testing_hdrs = [
 
 storage_client_testing_srcs = [
     "testing/client_unit_test.cc",
-    "testing/mock_client.cc",
     "testing/mock_http_request.cc",
     "testing/object_integration_test.cc",
     "testing/random_names.cc",
diff --git a/google/cloud/storage/testing/mock_client.cc b/google/cloud/storage/testing/mock_client.cc
deleted file mode 100644
index 88cd26aba3f3..000000000000
--- a/google/cloud/storage/testing/mock_client.cc
+++ /dev/null
@@ -1,29 +0,0 @@
-// Copyright 2023 Google LLC
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//     https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#include "google/cloud/storage/testing/mock_client.h"
-
-namespace google {
-namespace cloud {
-namespace storage {
-namespace testing {
-
-Client UndecoratedClientFromMock(std::shared_ptr<MockClient> mock) {
-  return internal::ClientImplDetails::CreateWithoutDecorations(std::move(mock));
-}
-
-}  // namespace testing
-}  // namespace storage
-}  // namespace cloud
-}  // namespace google
diff --git a/google/cloud/storage/testing/mock_client.h b/google/cloud/storage/testing/mock_client.h
index 6a763d82c63f..657beeca1402 100644
--- a/google/cloud/storage/testing/mock_client.h
+++ b/google/cloud/storage/testing/mock_client.h
@@ -203,7 +203,9 @@ Client ClientFromMock(std::shared_ptr<MockClient> const& mock,
  *
  * This client does not retry on transient errors.
  */
-Client UndecoratedClientFromMock(std::shared_ptr<MockClient> mock);
+inline Client UndecoratedClientFromMock(std::shared_ptr<MockClient> mock) {
+  return internal::ClientImplDetails::CreateWithoutDecorations(std::move(mock));
+}
 
 }  // namespace testing
 }  // namespace storage