Leonard Mann attended Moreland State School before winning a scholarship to Wesley College. Mann left school at sixteen to work as a clerk in the Defence Department and began reading for the Bar. He served in the 39th Battalion, 1st AIF during World War I, experiencing four years of trench warfare before returning to Melbourne to complete his law degree at the University of Melbourne. For most of the 1930s he was Secretary of the Employers' Federation, then held a senior position in the aircraft industry during World War II. After retiring, Mann kept a small farm in the Dandenong Ranges and later moved to coastal Inverloch in South Gippsland.
Mann is most admired for Flesh in Armour (1932), hailed as one of Australia's best novels of the soldier's experience of World War I. Initially Mann could not attract a publisher and arranged for private publication, but following the novel's critical success, he found it easier to attract publishers. In the next forty years Mann published seven novels, of which A Murder in Sydney (1937) and The Go-Getter (1942) became the best-known. Among other themes, Mann's fiction explores the struggles of small communities, the attraction of corruption during the Depression and the plight of Aborigines. While largely remembered for his fiction, Mann also published four volumes of poetry, winning the Grace Leven Prize for Elegiac and Other Poems (1957).
The autobiographical piece, 'A Double Life', written for the 'Australian Writers in Profile' series (Southerly vol. 29 no. 3, 1969), is a thoughtful account of a man who combined a life in industry with a literary life. After the death of his wife in 1976, Mann returned to the Dandenongs to live with his daughter. He died there in 1981.