A member of Flintshire County Council between 1995 and 1997, Thomas was elected to Parliament at the 1997 general election. Before 1997, Clwyd West had been regarded as a safe Conservative seat. His political interests include Wales, legal reform, social security and the environment. He was a member of the Select Committee on Welsh Affairs, the Select Committee on Social Security and the Joint Human Rights Committee. Following the 2001 general election he was appointed Parliamentary Private Secretary to the Secretary of State for Wales, Paul Murphy, and in October 2002 moved with Murphy to the Northern Ireland Office in the same capacity. He was a member of Labour's Parliamentary Campaign Team.