Leonard was born in October 1923 in Liverpool, Merseyside to Russian father Joel and Dora nee Barnett and he had one older sister Janette. Their father worked as a licensed victualler on Mulberry Street in Liverpool. Leonard attended King David high school before he enlisted and he is remembered on the school War Memorial.

Leonard was an engineer officer on board the S.S. New York, a former British passenger ship. The ship was torpedoed by U-96 south east of Cape Farewell on 25th September 1942. Most of the crew managed to evacuate the ship and were rescued by HMS Veteran. Sadly, HMS Veteran was torpedoed later the same day with the loss of all hands.