Leonard was born November 1921 in Hackney, London to Russian born Myer and Leah nee Rose. Myer and Leonard both worked as bakers in Hackney.

Leonard was the pilot of Mosquito DZ588 which departed from RAF Haverfordwest, Pembrokeshire on a training exercise. The aircraft was seen with black smoke coming from one engine and shortly after it banked and dived into a field at Little Treffgarne, Pembrokeshire killing Leonard and his navigator Warrant Officer Roy William Crack, 1394222.