diff --git a/packages/catalog-client/src/testUtils/InMemoryCatalogClient.test.ts b/packages/catalog-client/src/testUtils/InMemoryCatalogClient.test.ts
new file mode 100644
index 0000000000..0b5f0569bb
--- /dev/null
+++ b/packages/catalog-client/src/testUtils/InMemoryCatalogClient.test.ts
@@ -0,0 +1,109 @@
+/*
+ * Copyright 2024 The Backstage Authors
+ *
+ * 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
+ *
+ *     http://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.
+ */
+
+import { InMemoryCatalogClient } from './InMemoryCatalogClient';
+import { Entity } from '@backstage/catalog-model';
+
+const entity1: Entity = {
+  apiVersion: 'v1',
+  kind: 'CustomKind',
+  metadata: {
+    namespace: 'default',
+    name: 'e1',
+    uid: 'u1',
+  },
+};
+
+const entity2: Entity = {
+  apiVersion: 'v1',
+  kind: 'CustomKind',
+  metadata: {
+    namespace: 'default',
+    name: 'e2',
+    uid: 'u2',
+  },
+};
+
+const entities = [entity1, entity2];
+
+describe('InMemoryCatalogClient', () => {
+  it('getEntities', async () => {
+    const client = new InMemoryCatalogClient({ entities });
+    await expect(client.getEntities()).resolves.toEqual({ items: entities });
+  });
+
+  it('getEntitiesByRefs', async () => {
+    const client = new InMemoryCatalogClient({ entities });
+    await expect(
+      client.getEntitiesByRefs({
+        entityRefs: [
+          'customkind:default/e2',
+          'customkind:missing/missing',
+          'customkind:default/e1',
+        ],
+      }),
+    ).resolves.toEqual({ items: [entity2, undefined, entity1] });
+  });
+
+  it('queryEntities', async () => {
+    const client = new InMemoryCatalogClient({ entities });
+    await expect(client.queryEntities()).resolves.toEqual({
+      items: entities,
+      totalItems: 2,
+      pageInfo: {},
+    });
+  });
+
+  it('getEntityAncestors', async () => {
+    const client = new InMemoryCatalogClient({ entities });
+    await expect(
+      client.getEntityAncestors({ entityRef: 'customkind:default/e2' }),
+    ).resolves.toEqual({
+      rootEntityRef: 'customkind:default/e2',
+      items: [{ entity: entity2, parentEntityRefs: [] }],
+    });
+  });
+
+  it('getEntityByRef', async () => {
+    const client = new InMemoryCatalogClient({ entities });
+    await expect(
+      client.getEntityByRef('customkind:default/e2'),
+    ).resolves.toEqual(entity2);
+    await expect(
+      client.getEntityByRef('customkind:missing/missing'),
+    ).resolves.toBeUndefined();
+  });
+
+  it('removeEntityByUid', async () => {
+    const client = new InMemoryCatalogClient({ entities });
+    await expect(client.getEntities()).resolves.toEqual({
+      items: expect.arrayContaining([entity2]),
+    });
+    await expect(
+      client.removeEntityByUid(entity2.metadata.uid!),
+    ).resolves.toBeUndefined();
+    await expect(client.getEntities()).resolves.not.toEqual({
+      items: expect.arrayContaining([entity2]),
+    });
+  });
+
+  it('refreshEntity', async () => {
+    const client = new InMemoryCatalogClient({ entities });
+    await expect(
+      client.refreshEntity('customkind:default/e2'),
+    ).resolves.toBeUndefined();
+  });
+});
diff --git a/packages/catalog-client/src/testUtils/InMemoryCatalogClient.ts b/packages/catalog-client/src/testUtils/InMemoryCatalogClient.ts
index cde018a55c..745273c6bc 100644
--- a/packages/catalog-client/src/testUtils/InMemoryCatalogClient.ts
+++ b/packages/catalog-client/src/testUtils/InMemoryCatalogClient.ts
@@ -49,14 +49,14 @@ export class InMemoryCatalogClient implements CatalogApi {
   #entities: Entity[];
 
   constructor(options?: { entities?: Entity[] }) {
-    this.#entities = options?.entities ?? [];
+    this.#entities = options?.entities?.slice() ?? [];
   }
 
   async getEntities(
     _request?: GetEntitiesRequest,
   ): Promise<GetEntitiesResponse> {
     // TODO(freben): Fields, filters etc
-    return { items: this.#entities };
+    return { items: this.#entities.slice() };
   }
 
   async getEntitiesByRefs(
@@ -80,7 +80,7 @@ export class InMemoryCatalogClient implements CatalogApi {
   ): Promise<QueryEntitiesResponse> {
     // TODO(freben): Fields, filters etc
     return {
-      items: this.#entities,
+      items: this.#entities.slice(),
       totalItems: this.#entities.length,
       pageInfo: {},
     };
diff --git a/plugins/catalog-node/src/testUtils/catalogServiceMock.test.ts b/plugins/catalog-node/src/testUtils/catalogServiceMock.test.ts
new file mode 100644
index 0000000000..47850dd600
--- /dev/null
+++ b/plugins/catalog-node/src/testUtils/catalogServiceMock.test.ts
@@ -0,0 +1,63 @@
+/*
+ * Copyright 2024 The Backstage Authors
+ *
+ * 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
+ *
+ *     http://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.
+ */
+
+import { Entity } from '@backstage/catalog-model';
+import { catalogServiceMock } from './catalogServiceMock';
+
+const entity1: Entity = {
+  apiVersion: 'v1',
+  kind: 'CustomKind',
+  metadata: {
+    namespace: 'default',
+    name: 'e1',
+    uid: 'u1',
+  },
+};
+
+const entity2: Entity = {
+  apiVersion: 'v1',
+  kind: 'CustomKind',
+  metadata: {
+    namespace: 'default',
+    name: 'e2',
+    uid: 'u2',
+  },
+};
+
+const entities = [entity1, entity2];
+
+describe('catalogServiceMock', () => {
+  it('exports the expected functionality', async () => {
+    const emptyFake = catalogServiceMock();
+    const notEmptyFake = catalogServiceMock({ entities });
+
+    await expect(emptyFake.getEntities()).resolves.toEqual({ items: [] });
+    await expect(notEmptyFake.getEntities()).resolves.toEqual({
+      items: entities,
+    });
+
+    const mock = catalogServiceMock.mock();
+    expect(mock.getEntities).toHaveBeenCalledTimes(0);
+    expect(mock.getEntities()).toBeUndefined();
+    mock.getEntities.mockResolvedValue({ items: entities });
+    await expect(mock.getEntities()).resolves.toEqual({ items: entities });
+
+    const mock2 = catalogServiceMock.mock({
+      getEntities: async () => ({ items: [entity1] }),
+    });
+    await expect(mock2.getEntities()).resolves.toEqual({ items: [entity1] });
+  });
+});