The division was constituted in 1921 and assigned to the First Army. Not all authorized units were active during the division's lifetime. The division did not participate in the First Army maneuvers in 1935, 1939, and 1940. The 21st was inactivated on 1 October 1940 and disbanded on 1 November 1940. The division staff personnel were reassigned to the serve as the Brigade Staff for the 102nd Coast Artillery Brigade (Anti-Aircraft) and the Headquarters Battery was manned by personnel from the division headquarters troop and the 51st Cavalry Brigade. Major General Nathaniel Hillyer Egleston (New York Army National Guard) gave up command of the 21st and assumed command of the 102nd on 5 September 1940