Williams was called to the bar (Inner Temple) in 1974 and made a bencher in 2007. He practised in Cardiff from 1974 to 1988 and in London from 1988 to 2004. He became a Queen's Counsel in 1992, and served as a recorder until his appointment as a specialist Chancery judge for Wales in 2004. On 11 January 2007, Williams was appointed a High Court judge,[3] receiving the customary knighthood, and assigned to the Queen’s Bench Division. He served as a presiding judge for the Wales Circuit and as Deputy Chairman of the Boundary Commission for Wales.[4]
He was appointed president of Welsh tribunals in December 2017,[5] and retired from the post on 31 March 2023.[6]
He is active in several organisations, being president of Pendyrus Male Choir, and is closely associated with Tylorstown RFC, the rugby union club for which he played as a youth. His connection with rugby was furthered in 2012 when he was appointed as an unpaid independent chairman of the Professional Regional Game Board, an organisation set up by the Welsh Rugby Union to restructure the sport in Wales.[9]