He first moved to rugby union in 2010 and played mainly as a centre for Toulon in France, Canterbury, Counties Manukau, the Crusaders, Chiefs and Blues in New Zealand and Panasonic Wild Knights in Japan.

[20] As 2004 NRL premiers, Canterbury-Bankstown faced Super League IX champions, the Leeds Rhinos, in the 2005 World Club Challenge, which Canterbury lost 32–39.

[26] Speculation surrounding Williams's playing future ended when he re-signed with Canterbury on 9 March 2007 for a five-year contract worth over $2.5 million, extending through to the 2012 season.

[39] On 1 April, Williams scored his second try in the Roosters' 50–0 win over the Eels—their biggest ever victory against Parramatta and the first time they had kept their opposition scoreless in consecutive matches since 1999.

[43] On 28 July, Williams scored his seventh try against Newcastle, but he was given a two match suspension for a grade three careless high tackle on former Canterbury-Bankstown Bulldogs teammate Willie Mason.

[65] During his recovery he was a guest host on the Nine Network's Footy Show[66] and was part of a high-profile delegation to launch the NRL's Pacific Strategy in Samoa.

Williams announced his retirement via Instagram in early 2021, days after former Crusaders and dual World Cup-winning All Black teammate Dan Carter.

In July 2008, after linking up with new manager Khoder Nasser,[75] Williams left Australia to join the Tana Umaga coached French rugby union club Toulon.

[83] The circumstances in which he left the Bulldogs created a media debate in Australia and New Zealand, and Williams was the subject of considerable criticism for a departure that was described as the greatest act of treachery in the game's history.

[31] Williams rejected Toulon's offer, reportedly the largest in rugby union history, and opted to sign with the NZRU in a bid to play for the All Blacks.

[105] A week later, against the Stormers in Cape Town, he was part of the Crusaders team that became the first side since 1999 to win a Super Rugby semi-final outside their home country.

[106] On 9 July, Williams was part of the Crusaders team which lost to the Reds in the grand final, held at Suncorp Stadium before an Australian provincial attendance record crowd of 52,113.

[124] On 4 August, Williams played in the Chiefs' 37–6 victory against the Sharks in the Super Rugby final, scoring the last try of the match then celebrating by leaping into the home crowd.

[135] On 9 September, a week after sitting out the Top League season opener to be given time to adjust after coming off recent All Blacks duties,[136] Williams made his debut for Panasonic against NTT Communications Shining Arcs.

[138] On 27 October, Williams scored his second try during Panasonic's eighth round victory over the Kintetsu Liners;[139] however, the match would be his final game of the season after sustaining an injury when he landed awkwardly on his shoulder following a tackle.

[141] On 15 June 2014, Williams signed a two-year deal, starting in 2015, to play in the ITM Cup with Counties Manukau—reuniting with his former Toulon coach Tana Umaga.

[142] However, on 8 October 2014 he started his contract with Counties a year early when he made his debut for the club against Auckland, only twelve days after his last game for the Roosters.

[188] His tournament was again cut short, by an ankle injury early in the quarter-final loss to the United States, with New Zealand finishing the World Series in third place.

After serving his four-match suspension for his red card, Williams returned for the first Bledisloe Cup match of the year, where he scored a try in the All Blacks' record 54–34 victory.

Although this was Williams' third World Cup, he lost his place as a regular starter to Chiefs midfielder Anton Lienert-Brown, being used as a replacement for a pool stage test against eventual Champions, South Africa, as well as two of the playoff matches, against Ireland and England.

[200] On 27 May 2009, Williams made his debut as a professional boxer on the undercard of close friend Anthony Mundine,[201] defeating Garry Gurr with a technical knockout (TKO) in the second round in Brisbane.

[203] On 29 January 2011, Williams fought in his third professional match against Australian Scott Lewis at the Gold Coast Convention Centre, in his first fight as the main event.

"[212][213][214] On 8 February 2012, Williams was supposed to fight Richard Tutaki for the vacant New Zealand Professional Boxing Association (NZPBA) Heavyweight Championship title at Claudelands Arena in Hamilton, after Shane Cameron vacated it to move down to the cruiserweight division.

[220] Williams went on to claim the title belt by technical knockout after a left hook and a series of further blows on Tillman forced referee Lance Revill to stop the fight in the first round.

The event marked the inaugural Footy Show Fight Night[231] and was broadcast, in a rare occurrence, live on free-to-air television channel GEM.

[234] On 26 June 2021, in his first professional bout in over six years, Williams defeated Waikato Falefehi in a unanimous points decision 57–56, at the Townsville Entertainment Centre in Queensland, Australia.

[235][236] On 23 March 2022, Williams defeated former professional Australian rules footballer Barry Hall at Aware Super Theatre in Sydney via technical knockout (TKO) in the first round.

Williams lost to Mark Hunt in the fourth round, of an eight-round heavyweight bout on November 5, 2022, at Ken Rosewall Arena in Sydney, Australia.

[243][244] In December 2015, Williams served as an ambassador for UNICEF, visiting Lebanon with Mike McRoberts in an effort to raise awareness of the plight of Syrian refugee children.

[254] In 2019, he made a "tearful message of condolence" for the victims of the Christchurch mosque shootings[255] and denounced the treatment of the Uyghurs in China: "It's a sad time when we choose economic benefits over humanity.