Leslie Mawson was born in Idle, Yorkshire, on 9 July 1924, the son of Thomas Gilbert and Agnes Mawson. It is not known when he joined the Royal Navy or when he joined submarines, but he is reported to have served in HMS H34. Mawson had been ill since May 1944 and had recently returned home, from hospital at Liverpool. He was suffering from a duodenal ulcer, which brought about a nervous condition. On 20 September 1944, it is understood that Leslie Mawson died at home in Shipley, Yorkshire, from coal gas poisoning having committed suicide.

Leslie Mawson is commemorated at the Bradford Crematorium on Panel No 1.