In June 1917, then Captain Richardson was assigned to the 2nd Cavalry at Fort Ethan Allen, Vermont and on July 9, 1917, he was appointed Aide to Major General T. H. Barry who commanded the Central Department, Chicago, Illinois. He helped in the rapid buildup and training of the American Expeditionary Force (AEF) that was preparing to go to France. Then (temporary) Major Richardson sailed with General Barry to France from New York December 1, 1917. Fluent in French, Richardson served as Aide and observer with foreign armies until January 9, 1918. On June 14, 1918, he was assigned to the Operations Division, General Staff, AEF as Liaison Officer for G.H.Q Allied Headquarters and with American Armies, Corps, and Divisions during the combat operations of 1918. He escorted Allied missions in St. Mihiel Offensive. By now a temporary Lt Colonel, Richardson was Liaison Officer with Headquarters, 1st Army for the opening of Meuse-Argonne Offensive and the Operations Officer Representative at Advance G.H.Q. With the end of hostilities, now a temporary colonel, Richardson joined the Reparations Board, Peace Commission, Paris from January 28 to February 28, 1919. As part of the Army occupation forces Colonel Richardson served on temporary duty at Headquarters, Third Army in Coblenz, Germany, and was attached to Headquarters, 10th French Army, Mayence, Germany until June 1, 1919.