In March 1918, Harmon went to France with F Troop, 2nd Cavalry. The 2nd Cavalry was the only Cavalry unit to go overseas during World War I, and B, D, F, and H Troops became the last horse-mounted U.S. Cavalry units to ever engage an enemy in combat. Harmon served in the Baccarat Sector, at Camp du Valdahon, the St. Mihiel Offensive, and the Meuse-Argonne Offensive. He served in France and Belgium after the war, and returned to the U.S. in June 1919 to become a student at the Fort Riley, Kansas Cavalry School.