Capturing their first premiership since 2002, the Roosters beat the Sea Eagles for the fourth time in 2013 - Williams leaving an indelible impression in what shapes as his final game in the code before a return to rugby union.
It also handed coach Trent Robinson the ultimate prize in his first crack at the top job, becoming just the sixth man to do so.
It was a decider that had it all - lead changes, momentum swings and even a penalty try - but it was the late salvo of Williams after a less than spectacular opening hour that will emerge as the most significant memory.
Having trailed by ten points just after halftime, the Roosters got back in the contest when Aidan Guerra crossed in the 55th minute.
Then Williams, who had spilled balls and pushed passes all night, stepped up with a brilliant offload for James Maloney - his pass to Anthony Minichiello dubious before Shaun Kenny-Dowall raced away to give the Roosters the lead.
Maloney then set up his third try of the night as the third of the Rosters' big off-season recruits, Michael Jennings, touched down on a grubber from his five-eighth, his conversion pushing the lead beyond a converted try.
It was obvious from the first tackle when Glenn Stewart came reeling out in pain that it was all on the line, the nerves of a season full of expectation showing on an error-riddled Roosters.
Maloney put a kick out on the full and Williams coughed up the ball after a heavy hit from Kieran Foran - for which the Roosters paid the ultimate price as Jorge Tuafua scored in the ensuing set.
The Sea Eagles added a penalty goal to go out by a converted try, but the Roosters gathered their composure and got back into the contest - Maloney kicking to one wing and then the other for a flying Daniel Tupou to leap over David Williams to lock up the contest.
Things got heated but a head-butt from Jared Waerea-Hargreaves on Justin Horo went unpunished, the Rosters instead getting the penalty for a ruck infringement to gain their first lead of the match.
The calls weren't going the Sea Eagles' way, but all their luck came in one hit just after halftime when they were awarded a penalty try, the video referees unwilling to change on-field official Shayne Hayne's call that Jamie Lyon would have scored if not tackled without the ball by Mitch Aubusson.
Manly were now on top and full of running with Daly Cherry-Evans cutting them to shreds, Sonny Bill Williams poor start to the contest completed when a defensive misread allowed Steve Matai to go over for an 18-8 lead.
SYDNEY ROOSTERS 26 (A Guerra M Jennings S Kenny-Dowall D Tupou tries J Maloney 5 goals) bt MANLY 18 (J Lyon S Matai J Taufua tries J Lyon 3 goals).
Referees: Shayne Hayne, Ben Cummins.
Crowd: 81,491 at ANZ Stadium.