 Douglas-Mann was educated at Upper Canada College, Toronto, Ontario, Canada, and following national service in the navy, read PPE at Jesus College, Oxford from 1948 to 1951.In 1954, he qualified as a solicitor and served as a councillor on Kensington Borough Council 1962-65 and on the Royal Borough of Kensington and Chelsea from 1964.As a solicitor he specialised in trade union law and claims over industrial accidents and injuries.