Southampton Spitfires RLFC

Williams was a Marist Saints junior when he was spotted playing in Auckland by Bulldogs talent scout John Ackland. In 2002 he was offered a contract and moved to Sydney (as the youngest player to ever sign with the Bulldogs) to play in the Bulldogs junior grades. He advanced up the ranks quickly, becoming a starting player in the forward pack for the Bulldogs Jersey Flegg Cup side in his first year. The following year Williams cemented a starting spot in the Premier League side.

In 2004, when only 18 years old, he made his National Rugby League debut against the Parramatta Eels at Telstra Stadium. He experienced Premiership success in his rookie year and became the youngest person to play for the Bulldogs in a Grand Final[10] when playing off the bench in the Bulldogs' 16–13 victory over the Sydney Roosters in the 2004 Grand Final. Williams capped off a successful debut season by being named the International Newcomer of the Year Award and was named in Rugby League World magazine's 2004 World XIII.

In 2004, he was selected by New Zealand after only a handful of NRL matches and on 23 April made his debut for the Kiwis as their youngest-ever Test player in the 2004 ANZAC Test against Australia.[10] Williams played 15 NRL premiership matches during the season, firmly establishing himself in the Bulldogs squad. His contract was due to expire in 2005 and Williams reportedly received several lucrative offers to attempt to lure him away from the Bulldogs, with the largest rumoured to be about $3 million from Super League club St Helens in the UK.[13] Williams decided to stay with the Bulldogs and signed on for a further two years. St Helens chairman Eamonn McManus later said the club had not made an offer to him.

Williams had a shortened 2005 season after sustaining a severe knee injury plus several other minor injuries. Williams only played five games through the year and he subsequently missed several internationals for New Zealand. Williams publicly expressed his frustration, stating "You've got to be pretty strong mentally when you have injuries, and I've had a few."

Williams stayed relatively injury-free throughout the 2006 season, playing in 21 matches. He scored 8 tries and just missed out on a Grand Final berth, losing to eventual winners the Brisbane Broncos in the Preliminary Final. Despite having been mostly injury free throughout the year, off-season surgery forced him to miss the Tri-Nations for the Kiwis for the second year running. At the start of the 2007 NRL season, Williams' contract status was a frequent news item in the Australian print media. The speculation ended when Williams re-signed with the Bulldogs on 9 March 2007, with a 5 year contract believed to be worth over $2.5 million, that would have seen him stay with the club through to the 2012 season.

In the first game of the 2007 season, Williams was sent off and subsequently suspended for two weeks for a high tackle on Andrew Johns. In doing so, he became the first player of the 21st century to be sent off in a first-round game. He went on to play in 21 matches and score 14 tries during the season, although it did not end well as Williams broke his forearm in a tackle on Nathan Hindmarsh during the Semi Final against the Parramatta Eels. His team lost the match and Williams was again ruled out from representing his country in the post-season 2007 Great Britain Tour. He was nominated for 'Second-Rower of the Year' at the 2007 Dally M Awards but lost the award to Manly Warringah Sea Eagles second-rower Anthony Watmough.

Sonny Bill Williams in action