Shortly after the outbreak of World War II, he left school to enlist as a wireless operator and air gunner in the Royal Air Force. In 1942 he received a commission and the following year trained (in Canada) as a pilot. His operational career began late in 1944 when he joined 182 Squadron, flying Hawker Typhoons on close support to the Allied armies in the Low Countries. After serving also with 181 Squadron, he was given command of 182 Squadron shortly before the end of the war.
In 1947 Derry joined de Havilland as a test pilot, working largely on the de Havilland DH 108 aircraft. He is widely believed to have exceeded the speed of sound on 6 September 1948 when he lost control of his aircraft and the Mach meter briefly showed supersonic speeds in a shallow dive from 12,195 m (40,000 ft) to 9,145 m (30,000 ft), although recording apparatus was switched off.