In June 1918, McCormack enlisted in the United States Army for World War I, and was initially posted to Camp Devens, Massachusetts as a member of the 14th Company of the 151st Depot Brigade. After completing his initial training, McCormack was assigned to the Infantry Replacement Center at Camp Lee, Virginia to receive officer training. McCormack advanced through the ranks from private to sergeant major, and was attending Officer Training School at Camp Lee when the Armistice occurred. He was discharged in late November, following the end of the war.