diff --git a/frontend/src/__tests__/apikeys.test.ts b/frontend/src/__tests__/apikeys.test.ts
index ea5b09163..978bdd30c 100644
--- a/frontend/src/__tests__/apikeys.test.ts
+++ b/frontend/src/__tests__/apikeys.test.ts
@@ -927,5 +927,50 @@ describe('API Keys Module', () => {
       // The exact format depends on locale, but it should contain date parts
       expect(container?.innerHTML).toContain('2024');
     });
+
+    // #492: last_used_at null renders "Never", not "0 min ago" or empty
+    test('renders Never for null last_used_at (issue #492)', async () => {
+      const mockKeys = [
+        {
+          id: 'key-1',
+          name: 'Unused Key',
+          key_prefix: 'abc123',
+          is_active: true,
+          created_at: '2024-01-15T10:00:00Z'
+          // last_used_at intentionally absent
+        }
+      ];
+
+      (api.getApiKeys as jest.Mock).mockResolvedValue({ api_keys: mockKeys });
+      await loadApiKeys();
+
+      const container = document.getElementById('apikeys-list');
+      expect(container?.innerHTML).toContain('Never');
+    });
+
+    // #492: populated last_used_at renders relative time with ISO title for hover
+    test('renders relative time with ISO title for populated last_used_at (issue #492)', async () => {
+      const recentTs = new Date(Date.now() - 2 * 3600 * 1000).toISOString(); // 2 hours ago
+      const mockKeys = [
+        {
+          id: 'key-1',
+          name: 'Used Key',
+          key_prefix: 'abc123',
+          is_active: true,
+          created_at: '2024-01-15T10:00:00Z',
+          last_used_at: recentTs
+        }
+      ];
+
+      (api.getApiKeys as jest.Mock).mockResolvedValue({ api_keys: mockKeys });
+      await loadApiKeys();
+
+      const container = document.getElementById('apikeys-list');
+      // Relative label: ends with "h ago" for a 2-hour-old timestamp
+      expect(container?.innerHTML).toMatch(/\d+h ago/);
+      // Absolute ISO on the title attribute for hover
+      const span = container?.querySelector<HTMLElement>('[title]');
+      expect(span?.getAttribute('title')).toBe(new Date(recentTs).toISOString());
+    });
   });
 });
diff --git a/frontend/src/apikeys.ts b/frontend/src/apikeys.ts
index 0a1e8eb66..fb4633be8 100644
--- a/frontend/src/apikeys.ts
+++ b/frontend/src/apikeys.ts
@@ -4,7 +4,7 @@
 
 import * as api from './api';
 import type { APIKeyInfo, CreateAPIKeyResponse } from './types';
-import { formatDateTime } from './utils';
+import { formatDateTime, formatRelativeTime } from './utils';
 import { confirmDialog } from './confirmDialog';
 import { showToast } from './toast';
 import { openModal, closeModal } from './modal';
@@ -85,7 +85,7 @@ export function renderApiKeysList(): void {
               <td><code>${escapeHtml(key.key_prefix)}...</code></td>
               <td><span class="badge ${statusClass}">${statusText}</span></td>
               <td>${formatDateTime(key.created_at)}</td>
-              <td>${key.last_used_at ? formatDateTime(key.last_used_at) : '<span class="text-muted">Never</span>'}</td>
+              <td>${key.last_used_at ? `<span title="${escapeHtml(new Date(key.last_used_at).toISOString())}">${escapeHtml(formatRelativeTime(key.last_used_at))}</span>` : '<span class="text-muted">Never</span>'}</td>
               <td>${key.expires_at ? formatDateTime(key.expires_at) : '<span class="text-muted">Never</span>'}</td>
               <td>
                 ${key.is_active && !isExpired ? `<button class="btn-small btn-warning revoke-key-btn" data-key-id="${escapeHtml(key.id)}">Revoke</button>` : ''}
diff --git a/internal/auth/service_apikeys_api_test.go b/internal/auth/service_apikeys_api_test.go
index 74541265b..9ec8db7a1 100644
--- a/internal/auth/service_apikeys_api_test.go
+++ b/internal/auth/service_apikeys_api_test.go
@@ -151,6 +151,68 @@ func TestService_ListUserAPIKeysAPI(t *testing.T) {
 		mockStore.AssertExpectations(t)
 	})
 
+	// #492: last_used_at must round-trip through ListUserAPIKeysAPI so the
+	// frontend can render "Last used" without a separate endpoint.
+	t.Run("last_used_at is present in response when key was used (issue #492)", func(t *testing.T) {
+		mockStore := new(MockStore)
+		t.Cleanup(func() { mockStore.AssertExpectations(t) })
+		service := &Service{store: mockStore}
+
+		createdAt := time.Now()
+		usedAt := createdAt.Add(-2 * time.Hour)
+		keys := []*UserAPIKey{
+			{
+				ID:         "key-used",
+				UserID:     "user-123",
+				Name:       "Used Key",
+				KeyPrefix:  "prefix1",
+				IsActive:   true,
+				CreatedAt:  createdAt,
+				LastUsedAt: &usedAt,
+			},
+		}
+		user := &User{ID: "user-123", Email: "test@example.com", Active: true}
+		mockStore.On("GetUserByID", ctx, "user-123").Return(user, nil)
+		mockStore.On("ListAPIKeysByUser", ctx, "user-123").Return(keys, nil)
+
+		result, err := service.ListUserAPIKeysAPI(ctx, "user-123")
+
+		require.NoError(t, err)
+		resp := result.(*APIListAPIKeysResponse)
+		require.Len(t, resp.APIKeys, 1)
+		require.NotNil(t, resp.APIKeys[0].LastUsedAt, "last_used_at must be set in the API response")
+		assert.True(t, resp.APIKeys[0].LastUsedAt.Equal(usedAt))
+	})
+
+	// #492: last_used_at must be nil (not zero-time) for a never-used key.
+	t.Run("last_used_at is nil in response for never-used key (issue #492)", func(t *testing.T) {
+		mockStore := new(MockStore)
+		t.Cleanup(func() { mockStore.AssertExpectations(t) })
+		service := &Service{store: mockStore}
+
+		keys := []*UserAPIKey{
+			{
+				ID:        "key-new",
+				UserID:    "user-123",
+				Name:      "New Key",
+				KeyPrefix: "prefix2",
+				IsActive:  true,
+				CreatedAt: time.Now(),
+				// LastUsedAt intentionally nil
+			},
+		}
+		user := &User{ID: "user-123", Email: "test@example.com", Active: true}
+		mockStore.On("GetUserByID", ctx, "user-123").Return(user, nil)
+		mockStore.On("ListAPIKeysByUser", ctx, "user-123").Return(keys, nil)
+
+		result, err := service.ListUserAPIKeysAPI(ctx, "user-123")
+
+		require.NoError(t, err)
+		resp := result.(*APIListAPIKeysResponse)
+		require.Len(t, resp.APIKeys, 1)
+		assert.Nil(t, resp.APIKeys[0].LastUsedAt, "last_used_at must be nil for a never-used key")
+	})
+
 	t.Run("return empty list when no keys", func(t *testing.T) {
 		mockStore := new(MockStore)
 		service := &Service{store: mockStore}