Elworthy was granted a commission as a pilot officer in No. 600 (City of London) (Fighter) Squadron, part of the Auxiliary Air Force, where he flew Harts, with effect from 15 January 1935. He became attached to the Royal Air Force on 28 October 1935 and joined No.15 Squadron at RAF Abingdon, again flying Harts, with effect from the same date. He was granted a permanent commission in the Royal Air Force on 3 March 1936, promoted to flying officer on 3 September 1936 and became Personal Assistant to Air Chief Marshal Sir Edgar Ludlow-Hewitt, the Air Officer Commanding-in-Chief at RAF Bomber Command, in November 1937. Promoted to flight lieutenant on 3 September 1938, he was posted as a pilot and then a flight commander with No. 108 Squadron at RAF Bassingbourn flying Blenheim bombers in January 1939.