Leonard Arthur Thomson was born at Morwell, Victoria, on 13 December 1919. Before the Second World War he worked as a labourer and enlisted in the Australian Army at Royal Park, Victoria, on 14 July 1941. He served with the 2/10 Australian Infantry Battalion and later the 2/10 Field Workshops, seeing active service in New Guinea before being captured by the Japanese on 19 February 1942 during the fall of Singapore. Thomson spent more than three and a half years as a prisoner of war, enduring captivity until his liberation in September 1945. He was discharged from the Army on 17 January 1946. His wartime service later qualified him for the 1939–45 Star, Pacific Star, War Medal 1939–45 and Australian Service Medal 1939–45.

After the war, Leonard built a new life in civilian Australia, raising a family that later settled on Flinders Island.