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.

Williams was also one of the heroes of Wales's 2005 Grand Slam effort and was named the RBS Six Nations Man of the Championship, after a series of superb displays capped by the five-minute brace he scored against France.

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 toured Australia with the British and Irish Lions in 2001 and in 2005. He was a key member of the Welsh side in the 2003 Rugby World Cup, notably scoring a try and a drop goal against Tonga.

Williams's club career started at Pontypridd, but he has been at Cardiff since 1999. He made twenty appearances for his club during the 2006–2007 season and scored three tries, one of which sealed a first win for the Cardiff Blues on French soil in the Heineken Cup (against Bourgoin).

Williams was named in Gareth Jenkins's squad ahead of the 2006 Invesco Perpetual Series. He gained a try in the series opener against Australia and also in the finale against New Zealand, and further appeared in the victory over Canada. Williams starred in every game of the 2007 RBS Six Nations for Wales.

Having been rested from the Welsh squad to tour Australia in May 2007, Williams was named in Wales's 41-man preliminary Rugby World Cup 2007 summer training squad. In August he was included in the final thirty man cut, and played in both Invesco Perpetual Summer Series matches at the Millennium Stadium.

Williams made the starting line-up for all four of Wales's Pool B matches in the Rugby World Cup; he scored two tries in the 72-18 mauling of Japan at the Millennium Stadium and gained one try in the defeat to Fiji in Nantes.