Brain joined the Avon and Somerset Constabulary in 1978 under the graduate entry 'fast track' scheme, rising from Constable to Chief Inspector in that county before joining the Hampshire Constabulary on his promotion to Superintendent. He became Assistant Chief Constable in the West Midlands Police in 1994, where he was responsible for Community Affairs and later for Operations. His specific responsibilities included the policing of Euro ’96, counter terrorist operations, and the extensive reorganisation of the force in 1997. In 1998 he was promoted to become Deputy Chief Constable of Gloucestershire, where his responsibilities included community relations and strategic planning.