Leeman began work as a Junior Solicitors Clerk at the age of 13. He became a Solicitor and established a successful legal practice in 1835. He was a Liberal MP for York and Lord Mayor of York 3 times. Leeman was Deputy Chairman of the NER from 1855 to 1874 and Chairman 1874 to 1880. He was Chairman of the Yorkshire Banking Company and a shareholder in the York, Newcastle & Berwick Railway. He was Chairman of the Railway Association of Great Britain. He died in 1882 after a long period of ill health.