The Premier League has never witnessed a finale like it and could struggle to match the breathtaking twists and turns for years.
After 38 matches over nine months, all that separated City and United was goal differential as the clock wound down on the season Sunday evening.
"I don't want to spoil the moment," City captain Vincent Kompany said. "But I'm already looking forward to the next one."
The final dramatic turn — injury-time goals from Edin Dzeko and Sergio Aguero — swung the title race back in City's favour when all appeared to be lost.
United's title celebrations at Sunderland, where the deposed champions had won 1-0, were abandoned as manager Alex Ferguson hastily ordered his players off the field.
"I got the word during the game that City had scored to make it 2-2," United defender Rio Ferdinand said. "Then, after our game finished, it went flat and silent, with everyone waiting. The cheers from the home supporters let us know City had scored and the title had gone to them."
At the Etihad Stadium, dejected fans, who had exited when City was trailing 2-1 in the 90th minute, clambered over walls back into the stadium to join the party through a blue smoke-filled haze.
By then, the scoreboard read 3-2, and a group of players who hadn't been born when City last reached the pinnacle of English soccer in 1968 were still stunned that they had pulled off the most miraculous of comebacks.
"You want to say it's the best moment of your life," Kompany said. "But, if I'm honest, please never again this way."
It was, though, "a crazy finish for a crazy season," as manager Roberto Mancini said, sipping champagne.
A City title charge that at one stage had appeared unstoppable, looked to be over five weeks ago when United surged eight points clear and Mancini conceded the trophy.
The Italian hadn't factored in a United collapse.
"It's the hardest league in the world," United manager Alex Ferguson said. "And anyone that wins it deserves it."
As United has done 12 times in the 44 years since City's last championship triumph — all under Ferguson since 1993.
Even if City goes on to topple United as English soccer's dominant force, no title is likely to be recalled as fondly as its 2012 success.
The comeback for the ages was the result of more than $1 billion of investment over three years by Abu Dhabi's Sheikh Mansour.
"We changed the history of this club by beating a strong team like United," Mancini said. "I hope we can continue to win, but now we need to improve. It is the second time on the trot we have conceded the fewest goals. This year we also scored the most."
City's problem on Sunday, though, was waiting till the end to score.
United was leading on Wayne Rooney's goal when City went in front just before half time on a shot by Pablo Zabaleta that QPR goalkeeper Paddy Kenny fumbled into his own net.
But City gifted an equalizer to QPR at the start of the second half. Joleon Lescott's headed clearance went backward into the path of QPR striker Djibril Cisse, who drove a shot past goalkeeper Joe Hart.
On a day when nothing seemed to be going right for City, even the ejection of Joey Barton, a former City midfielder, failed to suppress QPR.
Barton lost his cool and threw an elbow at Carlos Tevez as a cross came into the penalty area. The QPR captain was seething after being ejected and kneed Aguero as he left the field. Players from both sides came to placate the raging player, who had to be dragged off the field in the 55th.
Still, QPR managed to go in front on a rare counterattack 11 minutes later, with the unmarked Jamie Mackie sending a downward header past Hart.
City then threw all of its attacking might forward.
Dzeko headed in the equalizer in the second minute of stoppage time, and Aguero weaved through the QPR defence before blasting the title-winning goal into the net two minutes later.
As Mancini raised his hands and ran along the touchline, Aguero — the son-in-law of Diego Maradona — tore his shirt off in jubilation before being mobbed by teammates.
"He was crying on the floor," Kompany said. "All the guys were pouring their eyes out. You don't see strong personalities like that showing their emotion so often.
"We expected to win the league today. The disappointment of being one goal down was incredible. For us to do it was one of the best moments of my life."
Rob Harris can be reached at http://twitter.com/RobHarrisSuggest a correction