After Harvard, Hamersley went to the Mexican border with Battery A of the Massachusetts National Guard, followed by a post as an ambulance driver for the American Field Service in France. While there, he attended the French Artillery School at Fontainebleau and, although the position of teaching horsemanship was offered to him, he joined the Sixth Field Artillery of the A.E.F's First Division as a lieutenant. He served in many engagements, and after the Armistice of 1918 was with the United States Army of Occupation in Germany for nine months. Hamersley later became a major in the Field Artillery Reserve and was a commander of the Jeff Heigl Post of the American Legion.