A 1-0 win over United saw City return to the top of the table on goal difference, and maximum points from their remaining games at Newcastle and at home to QPR will almost certainly seal the club's first title since 1968.
However as Manchester City's fans and players celebrated after a raucous title decider at Eastlands hyped as the biggest match in Premier League history, Mancini's eyes remained firmly on the prize.
"It is not finished," Mancini warned his players after Vincent Kompany's header clinched a famous win. "If we think it is finished, we will be making a big mistake."
"It is important that we are on the top now but it is more important to be there after May 13."
If City win their final two games, Manchester United would only be able to overhaul their bitter rivals by racking up big victories over Swansea and Sunderland. City's goal difference now stands at plus eight.
Monday's triumph capped a remarkable turnaround in the championship race, which appeared to be done and dusted as recently on April 8 when United went eight points clear after City's defeat at Arsenal.
But a disastrous run of results from United which has seen them take only four points from a possible 12 following one win, two defeats and a draw means the destiny of the title is now back in City's hands.
Mancini warned his players that United's stumble illustrated how quickly fortunes can change as they steeled themselves for their final two tests.
"Football is crazy," Mancini said. "It is important to always believe in yourself when you have a bad moment.
"We were on the top for 28 games and I think we deserve another chance.
"But in two games, anything can happen.
"It is enough for my players that they remember six weeks ago and we were four points ahead of United. In two games we lost everything. Up to now, we have done nothing."
United manager Sir Alex Ferguson had earlier been embroiled in a spectacular touchline bust-up with Mancini which saw the two managers need to be separated by the fourth official and staff from both clubs.
The pair clashed after Ferguson took exception to a rough challenge on Danny Welbeck by City hardman Nigel De Jong. Welbeck later hobbled away from Eastlands with his leg in a protective boot.
"He was out on that touchline the whole game haranguing the referee, the fourth official and the linesmen," Ferguson said of Mancini.
"The minute I come off the bench for a bad tackle by De Jong on Welbeck, he was out again. He can't have it both ways.
"He's been complaining about referees this season but he won't be complaining tonight that's for sure."
Ferguson had no complaints with Monday's defeat however and said his side were paying the price for blowing leads of 3-1 and 4-2 in their 4-4 draw with Everton at Old Trafford on April 22.
"You can look back at all sorts of games but you've got to look at the Everton one," he said.
"That was a ridiculous performance for the last seven minutes to lose two goals. That's maybe cost us the league."
United legend Gary Neville said Ferguson was now faced with the "worst nightmare" of losing the title on goal differenece.
"I know -- I've been in that changing room for 20 years -- it's his worst nightmare," Neville told Sky Television.
"Losing a championship on goal difference is what he always talks about, every single season.
"In January, in February, in March, whatever month it is going for a championship, he will say if you're 1-0, 2-0 up, 'score three, score four'.
"You never know whether it's going to get to goal difference.
"It is his worst nightmare."