diff --git a/Docs/Tasks.md b/Docs/Tasks.md
index 7b30165..7bd9d58 100644
--- a/Docs/Tasks.md
+++ b/Docs/Tasks.md
@@ -415,7 +415,9 @@ const source = timeRange === "24h" ? "fail2ban" : "archive";
 
 ---
 
-### TASK-021 — Persist sidebar collapse preference to `localStorage`
+### TASK-021 — Persist sidebar collapse preference to `localStorage` (done)
+
+**Where fixed:** `frontend/src/layouts/MainLayout.tsx`
 
 **Where found:** `frontend/src/layouts/MainLayout.tsx` — `useState(() => window.innerWidth < 640)` initialises collapsed state but never saves the user's toggle preference.
 
diff --git a/frontend/src/layouts/MainLayout.tsx b/frontend/src/layouts/MainLayout.tsx
index 88799fd..5501cae 100644
--- a/frontend/src/layouts/MainLayout.tsx
+++ b/frontend/src/layouts/MainLayout.tsx
@@ -40,6 +40,7 @@ import { useBlocklistStatus } from "../hooks/useBlocklist";
 
 const SIDEBAR_FULL = "240px";
 const SIDEBAR_COLLAPSED = "48px";
+const SIDEBAR_COLLAPSED_STORAGE_KEY = "bangui_sidebar_collapsed";
 
 const useStyles = makeStyles({
   root: {
@@ -220,23 +221,52 @@ export function MainLayout(): React.JSX.Element {
   const styles = useStyles();
   const { logout } = useAuth();
   const navigate = useNavigate();
-  // Initialise collapsed based on screen width so narrow viewports start
-  // with the icon-only sidebar rather than the full-width one.
-  const [collapsed, setCollapsed] = useState(() => window.innerWidth < 640);
+
+  const readSavedCollapsed = (): boolean => {
+    try {
+      const savedValue = localStorage.getItem(SIDEBAR_COLLAPSED_STORAGE_KEY);
+      if (savedValue === "true") {
+        return true;
+      }
+      if (savedValue === "false") {
+        return false;
+      }
+    } catch {
+      // Ignore storage errors and fall back to viewport heuristics.
+    }
+    return window.innerWidth < 640;
+  };
+
+  const [collapsed, setCollapsed] = useState<boolean>(readSavedCollapsed);
   const { status } = useServerStatus();
   const { hasErrors: blocklistHasErrors } = useBlocklistStatus();
 
   /** True only after the first successful poll and fail2ban is unreachable. */
   const serverOffline = status !== null && !status.online;
 
-  // Auto-collapse / auto-expand when the viewport crosses the 640 px breakpoint.
   useEffect(() => {
+    try {
+      localStorage.setItem(SIDEBAR_COLLAPSED_STORAGE_KEY, String(collapsed));
+    } catch {
+      // Local storage may be unavailable in some environments.
+    }
+  }, [collapsed]);
+
+  useEffect(() => {
+    const savedValue = localStorage.getItem(SIDEBAR_COLLAPSED_STORAGE_KEY);
+    if (savedValue !== null) {
+      return undefined;
+    }
+
     const mq = window.matchMedia("(max-width: 639px)");
     const handler = (e: MediaQueryListEvent): void => {
       setCollapsed(e.matches);
     };
+
     mq.addEventListener("change", handler);
-    return (): void => { mq.removeEventListener("change", handler); };
+    return (): void => {
+      mq.removeEventListener("change", handler);
+    };
   }, []);
 
   const toggleCollapse = useCallback(() => {
diff --git a/frontend/src/layouts/__tests__/MainLayout.test.tsx b/frontend/src/layouts/__tests__/MainLayout.test.tsx
index d24787b..ca53288 100644
--- a/frontend/src/layouts/__tests__/MainLayout.test.tsx
+++ b/frontend/src/layouts/__tests__/MainLayout.test.tsx
@@ -18,7 +18,7 @@ import { MainLayout } from "../../layouts/MainLayout";
 // Mocks
 // ---------------------------------------------------------------------------
 
-vi.mock("../../providers/AuthProvider", () => ({
+vi.mock("../../hooks/useAuth", () => ({
   useAuth: () => ({ logout: vi.fn() }),
 }));
 
@@ -56,6 +56,7 @@ function renderLayout(): void {
 describe("MainLayout", () => {
   beforeEach(() => {
     vi.clearAllMocks();
+    localStorage.clear();
   });
 
   it("renders the navigation sidebar", () => {
@@ -75,4 +76,17 @@ describe("MainLayout", () => {
     await userEvent.click(toggleButton);
     expect(screen.queryByText("BanGUI")).not.toBeInTheDocument();
   });
+
+  it("initialises sidebar collapsed state from localStorage", () => {
+    localStorage.setItem("bangui_sidebar_collapsed", "true");
+    renderLayout();
+    expect(screen.queryByText("BanGUI")).not.toBeInTheDocument();
+  });
+
+  it("persists sidebar collapsed state to localStorage when toggled", async () => {
+    renderLayout();
+    const toggleButton = screen.getByRole("button", { name: /collapse sidebar/i });
+    await userEvent.click(toggleButton);
+    expect(localStorage.getItem("bangui_sidebar_collapsed")).toBe("true");
+  });
 });