Allen enlisted in the U.S. Army, May 1976. He received basic training at Fort Leonard Wood, Missouri, attended Medical Equipment Maintenance training at Fitzsimmons Army Medical Hospital, Colorado, and was assigned to Reynolds Army Hospital, Fort Sill, Oklahoma. He was accepted for the advanced course in 1978, assigned to Letterman Army Medical Hospital, Presidio, San Francisco, California, where he was discharged November 1981 with the rank of Specialist 5.