Gregory was elected as a member of the Northern Ireland Assembly in 1998 representing the East Londonderry constituency and has been re-elected on every occasion since. He served as the Minister for Regional Development from July 2000 until September 2001. He also served as the Minister for Culture, Arts and Leisure from June 2008 until June 2009. He remained a member of the Assembly until he stepped down at the 2016 Assembly election.