He was assigned to the 36th Infantry Regiment in 1916 and was stationed on the Mexican border at Del Rio, Texas during the Pancho Villa Expedition. He was also promoted to the rank of first lieutenant dating back to the November 30, 1916. Subsequently, he was transferred to Fort Snelling, Minnesota, where he was appointed a company commander. He was also promoted to the rank of captain in the next year.
Gerow was promoted to the rank of major on September 26, 1918, and transferred to the Camp Sherman, Ohio, where he assumed command of 3rd Battalion of the 379th Infantry Regiment. Here Gerow assisted in training of newly activated 95th Infantry Division, which was prepared for deployment overseas. On 11 November, the Armistice with Germany was signed, ending the hostilities. The division's deployment was cancelled and it was demobilized in December 1918.
In January 1919, Gerow was called to Washington, D.C., where he was assigned to the Army Finance Office. His new task was to take charge of the payment of the Bonus granted by Congress to the all honourably discharged soldiers who had served between April 6, 1917, and November 11, 1918, in the World War. He stayed in this capacity until June 1920.
Interwar service
Gerow was subsequently sent overseas, where he was assigned to the American Expeditionary Forces, where he was appointed a commanding officer of the Visitors Bureau, stationed in Koblenz, Germany.