The Roosters, who finished the regular season at the top of the NRL ladder, trailed 18-8 midway through the second half but hit back to claim their first title since 2002.
After a slow start to the game Williams warmed to the occasion, producing the type of football that has put him in such high demand - running 16 times for almost 150 metres, producing four offloads and making a line break of his own.
Williams has completed his one-year contract with the Roosters and the 28-year-old is yet to disclose his next move, with a return to rugby union mooted.
"After it gets past a point in a season, if he decides not to stay, that's a huge hole you can't fill, you can't find that type of player at this time of year" Roosters coach Trent Robinson told reporters, adding that the club took a risk in bringing Williams back to rugby league.
Williams infamously walked out on a five-year contract with the NRL's Canterbury Bulldogs in 2008 to switch codes and join French club Toulon.
"He's proved a lot of people wrong this year. We've just decided to back him, we'll do the same again."
The Roosters' success was all the more impressive with Robinson in his first year as head coach of the club.
His team boasts big name players across the park, but Robinson believes this year has been about more than one player.
"We knew he had to work as one, we had to work hard, spend a lot of time together," he said.
"We felt like the pressure was on us and we were excited about that."
Manly halfback Daly Cherry Evans was awarded the Clive Churchill medal for a best on field performance despite being on the losing side.
His display will likely see him earn a spot in the Australian rugby league World Cup squad when it is named on Monday.
"Obviously a very bittersweet moment, I'd definitely swap that for a premiership ring," Cherry-Evans told reporters.
"It's been stereotyped to a winning side, so it was a very surprising moment."