The 21st Cavalry Division was constituted in the National Guard in 1921, originally allotted to the states of New York, Pennsylvania, New Jersey, and Virginia; and assigned to the First Army.

From 1922 to 1938, the division’s subordinate units generally held separate summer camps at locations within their respective states: the 51st Cavalry Brigade at Pine Camp, New York, for New York units; the 102nd Cavalry at Sea Girt, New Jersey; the 52nd Cavalry Brigade at Mount Gretna, Pennsylvania, 1921–35 and Indiantown Gap Military Reservation, 1936–40.

This allowed the brigade commander and his staff to work with both regiments to discern strengths and weaknesses of his units.

In March 1925, plans were developed to assemble the entire 21st Cavalry Division at Sea Girt for maneuvers.

The primary reason that the division did not play a part in those major maneuvers was that it did not have a staff organized, nor a commander appointed, until the summer of 1940.

Also that fall, the Secretary of War authorized the various states to begin final negotiations for the formation of the headquarters of the remaining National Guard cavalry divisions, less the 24th, which was already organized.