He was educated at Eton and Cambridge University. He missed his cricket blue, due to illness, but he was considered an exciting prospect before the Second World War broke out. He played in fifteen first-class matches as a right-handed batsman for Middlesex between 1939 and 1947, being awarded his county cap in 1946.
During World War II, Mann served as an officer in the Scots Guards. He won the Military Cross as an acting major. He was in command of "Left Flank" Squadron of the 3rd Tank Battalion Scots Guards, part of 6 Guards Tank Brigade, during the advance towards Sevenum, east of Eindhoven. Manns squadron attacked some well hidden anti-tank guns at night and forced them to withdraw.