He earned his first cap from the bench against France in 1999 Six Nations season and scored on his first full start for Wales with a try against Italy in the same season. He earned 73 caps for Wales and scored 53 international tries (51 for Wales, two for the British & Irish Lions) and 265 points (255 for Wales, 10 for the Lions). He is Wales’ most capped winger.
He was part of the Grand Slam-winning side in the 2005 Six Nations, where he scored tries against Italy, Scotland, and most famously England, the try that helped them achieve a famous 11–9 victory that got their campaign underway. He was then selected for the British & Irish Lions on their 2005 New Zealand tour. On 28 June, he equalled a single-game Lions record by scoring five tries in a tour match against Manawatu.
In December 2008 he won the BBC Welsh Sports Personality of the Year and in the following April, Shane was named as a member of the British & Irish Lions for their tour to South Africa. On 13 February 2010, Williams scored the winning try against Scotland in Cardiff. It was his 49th try for Wales (51st in all), and his 18th in the Six Nations – equaling the record achieved by a Welshman in the tournament, with Gareth Edwards.