 He volunteered for service in the US Army in June 1941, serving four and a half years in the Army Air Service.During World War II, Clark trained at Luke Field in Arizona then volunteered to serve in the Glider Pilot Corps, while his brother Joseph was in the Merchant Marine.Clark's unit, the 442nd Troop Carrier Group, 303rd Squadron, trained at Kirtland, New Mexico and Fort Sumner using Piper TG-8 gliders then was sent to Europe in 1944 where he named his assigned glider "The Ellicott City Express".