Leslie was born in 1916 in Sunderland to Solomon Friedman and South African born Esther nee Asher and had two younger sisters Leah and Sylvia. Their mother Esther was born in Barberton, South Africa and travelled to England with her mother and siblings after her father Moses was killed in the Boer War. In 1939 the family were living together at 56 Suffolk Street, Sunderland where Solomon is listed as a window glazier. Leslie enlisted in the RAF in 1940.

Leslie was the navigator of Mosquito NT224 which departed from Banff flying in a formation of four Squadron aircraft, detailed to attack shipping in Alesund Harbour, Norway. The formation overshot the Harbour and were turning over Gessen airfield to go back to make an attack, when 15 to 20 ME 109s intercepted and attacked. Two Mosquitoes did not return to base from this mission, and it was assumed that they had been shot down in the ensuing combat. It was reported that one aircraft had crashed into the sea and the other one had one engine on fire. Following post war enquiries and investigations, it was recorded in 1949 that the missing crew members had lost their lives at sea. Leslie was killed alongside Flying Officer William Nathan Cosman, DFC, RCAF J/23397.