Williams was Liberal Democrat spokesperson on public health, further education, schools and innovation, universities and skills. He served on two House of Commons select committees - the Education and Skills Select Committee and the Public Accounts Committee. He was also Chair of the Liberal Democrat Treasury Backbench Committee. 
Before entering politics, Williams worked as a tax consultant and is a Member of the Chartered Institute of Taxation. Williams also served on Avon County and Bristol City Councils, and was elected as councillor for Cabot ward in 1993 aged 26. He was Leader of the Bristol Liberal Democrat group from 1995 to 1997. 
He attended Mountain Ash Comprehensive School and the University of Bristol, graduating in 1988 with a degree in history.