He joked that he would turn up for the press conference naked if he won this year's World Championship, and true to his word, he strode into the media room wearing just a towel, which he removed when shielded by a table.
It's the third time the Welshman has won the championship.
And after saying earlier in the tournament that he would go naked to his press conference if he won the title, Williams walked into his press conference undressed apart from a towel.
At 43, Williams became the oldest victor of the sport's biggest prize since fellow Welshman Roy Reardon won his sixth and final world title in 1978.
The 43-year-old, who had promised to strip off if he won, beat Scotland's John Higgins 18-16 in a dramatic final to become the event's oldest victor for 40 years.
Williams has now promised to perform "naked cartwheels" if he manages to defend his world title next year.
Higgins said: "I made it 17-16 but then he made one of the best breaks I've ever seen under pressure".
He said: "Last year I wasn't even here (he lost in qualifying)".
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Incredibly none of Williams' three children were born the last time he lifted the trophy 15 years ago - the biggest gap between world titles.
He led 10-7 overnight and soon stretched that to 14-7, Higgins potting just 31 points over the four frames. Twelve months ago I was thinking about chucking it and here I am as the 2018 world champion, bollock naked!
Williams did not make the Crucible past year and said: 'It's unbelievable. I've just done something I never thought I would.
Welshman Mark Williams pinched himself in amazement after withstanding a stoic fightback from John Higgins to seal his third World Championship title.
"I made a really good break under pressure".
Back came Williams, and a century break to go one frame away from victory suggested his nerves had settled.
'I knew at the end if I didn't get enough he was going to clear up again. Frame 30 followed a familiar pattern as Williams got in first and made 47 before missing a red to a top corner, and Higgins ruthlessly punished him with a 62 clearance to square the match for the first time since 7-7. I didn't want to lose with a session to spare, and at 14-7 that looked possible.