Williams was also one of the stars of the 2005 RBS Six Nations campaign, touching down against England, Italy and Scotland. On the 2004 summer tour to Argentina and South Africa he scored a hat-trick in the second test against Argentina. Furthermore, in the 2006 summer tour to Argentina Williams played in both Tests, notching a try in the second.

Williams started his career as a scrum half at Amman United, later moving to Neath and to the wing. He currently plays regional rugby with the Ospreys, having moved into the side with the inception of regionalism. To date Williams has made 68 appearances for the region and scored 27 tries and one drop goal.

Williams scored in his first full start for Wales with a try against Italy in the 2000 Lloyds TSB Six Nations and with 29 tries in 46 appearances, has one of the best strike rates in world rugby. He also played in the second test on the 2005 Lions Tour.

Williams was included in the Welsh squad for the 2006 Invesco Perpetual Series. He scored a superb try against the Australians in the first match of the Series, came on as a replacement against the Pacific Islands, scored a try in the third Test against Canada and also appeared in the series finale against the All Blacks.

The 2007 RBS Six Nations saw Williams partake in three games: the win over England and matches against France and Italy, in which he scored a try against the Azzurri. Williams was rested from the touring squad of  Australia in May 2007 but was named in Gareth Jenkins's 41-man preliminary Rugby World Cup 2007 summer training squad in June. He made the final 30-man RWC squad in August and played against France in the Invesco Perpetual Summer Series at the Millennium Stadium.

Williams was included in the starting line-up for all of Wales's Pool B matches at the Rugby World Cup, and enjoyed a successful personal tournament. He scored six tries in total, two against both Canada and Japan and one each against Australia and Fiji, making him the joint third highest try scorer of the entire tournament (on a par with Doug Howlett, behind Bryan Habana and Drew Mitchell). Williams missed out on a chance to play in the Prince William Cup clash against South Africa in November 2007 due to injury.

In January 2008 Williams was included in Warren Gatland's RBS Six Nations squad, and the winger proved a revelation in the championship. Williams amassed an impressive six tries, equalling Will Greenwood's Six Nations try-scoring record (in the Lloyds TSB Six Nations, 2001), playing in all five matches of the Grand Slam campaign and gaining braces against Scotland and Italy, and single efforts against Ireland and France.