In 2004, Sonny started the season in the NRL, making a dynamic debut against Parramatta where he scored a try before becoming a regular member of the team.

He was selected to play for New Zealand in his debut season and played a further six Tests for New Zealand from 2004 to 2007. He also played in the Premiership winning team against the Roosters from the bench, landing a bone-crunching shoulder charge on the Roosters Chris Flannery.

In 2005, Sonny's season was ruined by a knee injury. In 2006 he returned to mainly play centre due to injuries to Andrew Emelio and Willie Tonga. Sonny played in the Preliminary Final against Brisbane as a lock.

In 2007, Sonny was back to his damaging best for the Bulldogs. He was the top try scorer for the season with 14, one ahead of centre Daryl Millard. Williams played 21 games and a total of 1,351 minutes, topping the hitup count with 318. He produced the most offloads with an impressive total of 61, an average of three-per-game.

Sonny was at his best in round 22 against Canberra when he single handedly ripped the Raiders apart. He scored three tries and knocked Michael Dobson into last year with a bone crunching shoulder charge. The New Zealander led almost every statistic before he was replaced in the 60th minute to a standing ovation.

2008 saw the end of Sonny Bill's rugby league career when he left mid-season to join French Rugby Union club Toulon with a further four years remaining on his contract.

A month after leaving the Club a settlement was reach with Toulon and Sonny was released from his contract with the Bulldogs.