The Sydney Roosters were trailing 18-14 and Manly were eyeing a third premiership in five seasons.
But 30 metres in front of his own posts and 30 minutes remaining, the Kiwi superstar flicked a delicious inside pass to James Maloney.
The five-eighth burst through the Sea Eagles’ defence, ran 50 metres and found Anthony Minichiello with a pass that looked suspiciously forward.
The former NSW fullback then lobbed the ball into the grateful arms of Shaun Kenny-Dowall and the big centre scored.
Maloney’s conversion put the Roosters two points in front and left the Sea Eagles bitterly complaining to the officials.
But there was no arguments about the winning try seven minutes from time.
Williams saw a gap, broke through, and the ball once again found Maloney whose grubber kick was magnificently touched down by Michael Jennings.
Great players know when to pick their moments in the biggest games and Williams, in perhaps his last ever game of rugby league, showed once again he deserves all the plaudits that have come his way this season.
It was tough on Manly.
They were on the wrong side of several poor decisions by the officials in addition to Maloney’s forward pass.
Daly Cherry-Evans won the Clive Churchill Medal – only the third player to win it from a losing team – and was superb in tandem with halves partner Kieran Foran.
Battle-scarred Kiwi centre Steve Matai made one of the great grand final bell-ringers with a huge shot on Aidan Guerra and then scored the try that gave Manly an unassailable-looking 18-8 lead nine minutes after the restart.
But the Roosters showed why they have been the best side this year as they rallied to haul in the Sea Eagles’ advantage.
Aidan Guerra darted over seconds after the excellent Sam Moa was held up just short of the line by George Rose to start the fightback and Maloney, Williams and Jennings did the rest to secure a first premiership since 2002.