Constituted 24 June 1921 in the Organized Reserves as Headquarters and Headquarters Company, 199th Infantry Brigade, an element of the 100th Division. Organized in December 1921 with Headquarters at Huntington, West Virginia. Redesignated 23 March 1925 as Headquarters and Headquarters Company, 199th Brigade. Location changed 27 October 1931 to Parkersburg, West Virginia. Redesignated 24 August 1936 as Headquarters and Headquarters Company, 199th Infantry Brigade.
Converted and redesignated 23 February 1942 as the 100th Reconnaissance Troop (less 3rd Platoon), 100th Division. (Headquarters and Headquarters Company, 200th Infantry Brigade, concurrently converted and redesignated as the 3rd Platoon, 100th Reconnaissance Troop, 100th Division.) Troop ordered into active military service 15 November 1942 and reorganized at Fort Jackson, South Carolina as the 100th Cavalry Reconnaissance Troop, an element of the 100th Infantry Division. Reorganized and redesignated 2 August 1943 as the 100th Reconnaissance Troop, Mechanized. Reorganized and redesignated 7 September 1945 as the 100th Mechanized Reconnaissance Troop. Inactivated 11 January 1946 at Camp Patrick Henry, Virginia.