Mosley's political career began in 2000 when he was elected to Chester City Council. He was re-elected in 2004. He served as Executive Member for Customer Services. In 2005, he was elected to represent the Chester Overleigh Division on Cheshire County Council, and was Chairman of the Health & Adult Social Care Scrutiny Committee, Vice-chairman of the Community Services Scrutiny Committee and a member of the Cheshire Fire Authority.
During the House of Commons vote in October 2011 on a backbench motion for a bill to be introduced in the next parliamentary session to enable a referendum to be held on Britain's continued membership of the European Union, Mosley was one of 81 Conservative MPs who voted against the GovernmentWhip to support a referendum.