In April 1917, a few days after the United States entered World War I, Kilmer enlisted in the Seventh Regiment of the New York National Guard. In August, Kilmer was assigned as a statistician with the 165th Infantry Regiment (better known as the re-designated "Fighting 69th", U.S. 69th Infantry Regiment), of the 42nd "Rainbow" Division, and quickly rose to the rank of sergeant. Though he was eligible for commission as an officer and often recommended for such posts during the course of the war, Kilmer refused, stating that he would rather be a sergeant in the Fighting 69th than an officer in any other regiment.