Williams was capped at all age levels and is a former captain of the U21 side. He made his Wales senior debut against the Barbarians in 1996, and has captained his country on five occasions. Williams' club career started at Pontypridd, but he has been at Cardiff since 1999.

Williams toured Australia with the British & Irish Lions in 2001 and in 2005. He was a key member of the Welsh side in the 2003 Rugby World Cup.

Following Wales’ early exit from the 2007 Rugby World Cup, Williams announced his retirement from the international stage. However, following the appointment of new Head Coach Warren Gatland, Williams was persuaded out of retirement for inclusion in the Wales squad ahead of the 2008 RBS 6 Nations.

Williams enjoyed a hugely successful campaign; he featured in every match of the Grand Slam winning tournament, scored a try in the championship decider against France and was awarded the Man of the Match award for his superb performance against the Scots at the Millennium Stadium. He was also one of the heroes of Wales’ 2005 Grand Slam effort and was named the RBS 6 Nations Man of the Championship, after a series of superb displays.