During World War I, Conger served as assistant chief of staff for Intelligence (G-2) on the staff of the 2nd Division, and then as a member of the Intelligence directorate (G-2) on the staff of the American Expeditionary Forces headquarters. While on the AEF staff, Conger was the lead planner for a summer 1918 deception which became known as the "Belfort Ruse". This effort to deceive German commanders into allocating forces away from Saint-Mihiel was intended to produce an Allied advantage during an upcoming offensive. At the ensuing Battle of Saint-Mihiel, the Allied numerical superiority produced by Conger's deception surprised the Germans and caused a rapid Allied victory. After this success, Conger was assigned to command the 56th Infantry Brigade, which he led until the end of the war.