diff -u b/core/tests/Drupal/FunctionalJavascriptTests/Ajax/AjaxMaintenanceModeTest.php b/core/tests/Drupal/FunctionalJavascriptTests/Ajax/AjaxMaintenanceModeTest.php
--- b/core/tests/Drupal/FunctionalJavascriptTests/Ajax/AjaxMaintenanceModeTest.php
+++ b/core/tests/Drupal/FunctionalJavascriptTests/Ajax/AjaxMaintenanceModeTest.php
@@ -55,12 +55,7 @@
     $page = $this->getSession()->getPage();
     $assert_session = $this->assertSession();
 
-    // Turn on maintenance mode.
-    $edit = [
-      'maintenance_mode' => 1,
-    ];
-    $this->drupalGet('admin/config/development/maintenance');
-    $this->submitForm($edit, 'Save configuration');
+    \Drupal::state()->set('system.maintenance_mode', TRUE);
 
     $this->drupalGet('ajax-test/insert-inline-wrapper');
     $assert_session->pageTextContains('Target inline');