Before his political career Leskinen was a successful swimmer in the association of Helsinki Workers' Swimmers. He won the 200 and 400 meter breaststroke events at the 1937 Workers' Olympiad in Antwerp. Leskinen was later the Secretary General of the Finnish Workers' Sports Federation 1940-1943 and the chairman 1951–1955. Leskinen fought in the Winter and Continuation Wars. He was wounded in 1941. His military denomination was a Major.