Scouted by Arthur Beetson, Tonga moved into the Parramatta area. He was signed by the Parramatta Eels and played 8 games over two seasons. He then signed with the Canterbury-Bankstown Bulldogs.
Making small but sure career moves, his best-known achievements were yet to be acknowledged when he moved on to the Bulldogs in 2004. His career catapulted in that one year, earning State of Origin selection, allowing him to become the 145th player to be chosen for the Queensland side since 1980. Winning the Dally M Centre of the Year award, Tonga played there for the Bulldogs in their 2004 NRL Grand Final victory over cross-city rivals, the Sydney Roosters. Tonga was selected in the Australian team to go and compete in the end of season 2004 Rugby League Tri-Nations tournament. In the final against Great Britain he played at centre and scored two tries in the Kangaroos' 44–4 victory.
As 2004 NRL premiers, the Canterbury-Bankstown Bulldogs faced Super League IX champions, Leeds in the 2005 World Club Challenge. Tonga played at centre in the club's 32–39 loss. He was also picked for the Australian side for the 2005 Tri-Nations series, making his first full year in the NRL a success. On 1 April 2008, it was announced that Tonga had signed a four-year deal with the North Queensland Cowboys, commencing in 2009. Tonga's final year at Canterbury-Bankstown saw him play only seven games as the club finished last on the table.[8][9]
In April 2009, he was named in the preliminary 25 man squad to represent Queensland in the opening State of Origin match for 2009.[10] In June, Willie Tonga played in Game II of the 2009 State of Origin Series to replace Justin Hodges. Tonga was retained for the third match which the Maroons lost.
In 2010 Tonga played all three Origin matches for the Maroons. At the end of the season he also represented Australia again for the first time in five years in the 2010 Four Nations tournament.
After a number of long-term injuries, Tonga announced in 2011 that he would be re-joining the Parramatta Eels. This decision was welcomed by the fans and senior members of the Parramatta team who he had played with as young player.
Tonga's return to the club he debuted with was one of long term injuries, he struggled to get a game, only playing 12 in 2012 as the club finished last for the first time since 1972. In the 2013 preseason, during a trial match, Tonga injured his back and was ruled out indefinitely. In June 2013, Tonga was one of 12 Parramatta players that were told that their futures at the club were uncertain by coach, Ricky Stuart. Tonga returned towards the end of the 2013 NRL season making four appearances as Parramatta finished last for a second consecutive year.[11][12]
Tonga later re-signed with Parramatta until the end of the 2015 NRL season. In the 2014 NRL season, Tonga played 16 games as Parramatta narrowly missed the finals under new head coach Brad Arthur.[13]
In late 2013 it was revealed that Tonga's best friend Reni Maitua had been suffering from severe depression. In mid 2013, Maitua suffered a breakdown after a game in Brisbane, and days later attempted suicide. Tonga was awake at 1:00am when he received a call from Maitua's sister, who had received a text message which scared her greatly. Tonga chose to drive to Maitua's unit, and found Maitua attempting to take his own life. Tonga believes that had he even arrived 10 seconds later, Maitua would not have survived.[14] Tonga and Maitua told their story on the NRL Footy Show in June 2014 as an attempt to help remove the stigma around mental illness in league.
On 9 July 2014, it was confirmed that Tonga had signed a contract to play for the Super League club the Catalans Dragons in 2015.[15]
He made his Super League début in round 1 against the 2014 Champions, St. Helens. He scored his first ever Super League try against the Warrington Wolves in round 3.
