Hughes first joined the Liberal Party in 1971, when he signed up to Cambridge University Liberal Club as a student. As part of the SDP–Liberal Alliance, he was spokesman for the environment from 1983 to 1988. Along with the majority of Liberals, he joined the newly founded Liberal Democrats in 1988, acting as spokesman for education until 1992, then the environment again until 1994, then health until 1997, and then home affairs until 2003. He was the Liberal Democrat candidate in the 2004 Mayor of London elections and came in third with 15.22% of the first preference vote.