When the U.S. entered World War I, Leymel finished out teaching in the 1916-1917 school year before resigning to re-enlist in the U.S. Army. Leymel would report to the Presidio of San Francisco for Officer Training and was stationed at Camp Kearny before being sent to Europe. Leymel would serve in an artillery unit of the 91st Division and see action in northern France and southern Belgium in the Meuse–Argonne offensive