But there is still a hole in his sporting CV. A hole that can only be filled by five big rings.
"Going back in time the best sportsmen ever have been Olympians," said the 30-year-old as he announced he would dedicate himself to being part of rugby sevens' debut at Rio 2016.
"Usain Bolt, Muhammad Ali, the list goes on. It's just an amazing opportunity to become one of those."
This weekend is the start of that journey as Williams - a little more than a month after lifting the Webb Ellis Cup - heads to Dubai for the first event of the globe-trotting Sevens World Series.
This is not one of six World Series events that Williams aims to play in. Instead he has cut short his off-season to meet his new team-mates and learn the seven-a-side game from the sidelines.
And he is not the only 15-a-side star aiming to be part of rugby's new frontier.
Who might make the switch?
On his flight to the Middle East from Sydney, Williams bumped into Australia fly-half Quade Cooper, the pair tweeting pictures of themselves grinning away in business class.
Cooper was en route back to French club Toulon rather than stopping off in Dubai for the sevens. But he is determined to be on the plane to Rio.
His contract negotiations with Toulon's hard-ball president Mourad Boudjellal were long and laborious as Cooper successfully fought for the chance to play sevens for Australia alongside his club commitments.
When he lands in the south of France, Cooper might discuss his summer plans with new club team-mate Bryan Habana.
The Springboks leading try scorer, along with fellow World Cup stars Damian de Allende and Lwazi Mvovo, was invited to train with the South Africa sevens team in November as coach Neil Powell considers whether to import 15-a-side specialists into his Olympic side.
Cooper's international team-mates Bernard Foley and Israel Folau have opted for short stints in Japanese club rugby and will make a decision on whether to try their hand at sevens when they return early in 2016.
Winger Henry Speight, also part of the Wallabies squad at the World Cup, has already made the switch and is in the Australia squad for the Dubai event.
Flanker Liam Messam - another of the All Blacks World Cup-winning squad - has joined Williams in focusing solely on sevens this season.
And Argentina winger Santiago Cordero - one of the stars of the Pumas' run to the World Cup semi-finals - is likely to be part of their Rio effort after signing a central contract with his country.
Montpellier flanker Fulgence Ouedraogo and Castres winger Remy Grosso helped France secure their place at Rio in the summer before joining their country's Rugby World Cup squad.
All those format flip-floppers will have to contend with sevens specialists, men who understand the game's cagey, contact-averse tactics and lactic-heavy physical demands.
What of Team GB?
Wales wing George North has mused on the possibility of joining cyclist girlfriend Becky James in a British Olympic team.
Danny Cipriani is interested in being involved after his maverick talents were deemed surplus to England's World Cup requirements.
Full-back Stuart Hogg's electric running and courageous tackling for Scotland would surely transfer well to the seven-a-side game.
The names come easily. The reality is more complicated.
Great Britain qualified for Rio 2016 on the back of England's top-four finish in last season's Sevens World Series and the team will be made up of the best players from England, Scotland and Wales at Rio.
|Who could make the sevens grade?|
|Put together your composite sevens side from the best English, Welsh and Scottish 15-a-side specialists and share it with us via #rio7sstars.|
However, the chances of recruiting 15-a-side superstars to concentrate on sevens - as Williams has done - is a non-starter for Team GB as they don't have central contracts, while clubs are understandably keen their employees focus on their well-paid day jobs.
English club Wasps have indicated that they - and the rest of the Premiership - will adhere to the regulations and only release players two weeks before the tournament.
Barring an unforeseen bout of Olympic enthusiasm from clubs in the midst of trying domestic and European campaigns, Great Britain coach Simon Amor is likely to have to wait until the second-tier European Grand Prix Sevens series at least before he can welcome 15-a-side players into the fold.
The final leg of that series will take place in Exeter on 9-10 July and Great Britain are hoping their involvement will be confirmed before the end of this year.
Amor will then be faced with the unenviable task of condensing players from three different nations and two different formats into one team in less than a month.
Another gender, another planet
What Amor would give for the access that Simon Middleton, coach of Great Britain's women's team, has to 15-a-side specialists.
A week after England won the Women's Rugby World Cup in August 2014, the Rugby Football Union - mindful of the funding that potential Olympic medals bring to women's rugby - handed 12 of that squad and another eight players contracts to concentrate on sevens full-time.
Emily Scarratt, who scored 16 points in that 21-9 World Cup final win over Canada, is captaining England in Dubai, with another five of that historic squad also involved.
A Great Britain select seven - including Scots Steph Johnston and Megan Gaffney - also played in an invitational tournament running alongside the main Women's Sevens Series event.
Whoever is picked for Rio though, New Zealand will be overwhelming favourites for gold.
They have won all three of the Women's World Series titles since the it was introduced in 2012.
This season, they have a new recruit with Niall Williams, sister of Sonny Bill, making her debut in Dubai.
Elsewhere in Dubai
- Defending champions Fiji, coached by former England boss Ben Ryan, stepped in to serve food on one of the flights on their trip to Dubai while the air crew cared for an ill passenger.
- Craig Joubert - the referee who mistakenly awarded Australia a match-winning penalty in the Rugby World Cup quarter-final against Scotland - makes a return to high-profile international action for the first time since then as part of the team of officials in the Middle East.
- The death of former All Black Jonah Lomu - who came to prominence on the sevens circuit before his landmark 1995 World Cup campaign - will be marked with teams wearing black armbands for the first eight games on Saturday.
- Carlin Isles and Perry Baker - who have both transferred their phenomenal speed from American football - are eye-catching names in the United States squad.