Leonard was born in 1923 in Wandsworth to Russian born Joseph Kosminsky and Rachel nee Kennedy and had one younger sister Evelyn. The family surname was changed from Kosminsky to King. Their father died in 1934 aged thirty-nine. Research is ongoing to establish further details about his family.

Leonard enlisted into the RAF after December 1941 and details of his service are unknown to date. He died at Westminster Hospital, London after the end of the war and his death certificate states that his cause of death was Hodgkin’s Disease.