Leonard was born 17 September 1918 in Hendon, Middlesex to Polish born Simon and Hilda Fass and was the youngest of four children. Their father Simon originated from Poland and worked as a butcher in Hoxton. He died in 1938. In 1939, Leonard was living with his widowed mother and siblings. The family ran a butchers shop and Leonard was working as an engineers fitter.

Leonard was the observer on Blenheim R3806 which departed from RAF Thornaby. The aircraft failed to gain height after leaving the ground and flew into trees on the edge of the airfield. This caused to swing to starboard and to crash into the next field. Unfortunately before the crew could get clear the bomb load exploded and killed all three airmen in the aircraft. It was thought that the aircraft had not reached flying speed when the pilot attempted to lift the aircraft off the ground.

Those killed alongside Leonard were:
Pilot, Bruce Beardsley, 939008
W/Op,Lionel John Symes, 975686