Kelly dropped out of high school and lied about his age in order to enlist in the Army at the age of 15. He used a birth date of 22 December 1922 and enlisted under the name "Charles L. Kelley." He reported for active duty at Fort Screven, Georgia, on 25 February 1941 for 1 year of service as an army medic. As enlistment contracts at that time included an "or duration plus six months" clause, Kelly left the service as a corporal on 3 August 1945. He served overseas with the 30th Infantry Division and, although he was listed as a medic as late as May 1944, he was serving as an infantryman when he was wounded by an artillery shell fragment during the battle for Aachen, Germany. Kelly's injury was a severe one, a compound fracture of the fibula which kept him hospitalized for treatment and recovery from the time he was wounded in October 1944 until June 1945. Patrick Henry Brady, who served under Kelly in the Republic of Vietnam, said that Kelly's experience as a wounded infantryman was what would later drive him as a medical evacuation pilot.