Vincent Kompany scored the only goal as City, three weeks after appearing to crash out of the Barclays Premier League title race, regained the initiative at the top of the table.
The game may not have been the thriller the build-up had suggested and an estimated TV audience of 400million had craved. Yet with so much on the line, that was probably always too much to hope for.
What unfolded was still a compelling encounter with two great rivals slugging it out in the face of immense pressure. That was never more in evidence than on the touchline as managers Mancini and Ferguson needed to be parted in the second half as tensions boiled over.
It took a little while to reach that fervour, though, as this most hyped of encounters came to life only slowly.
The mood around the stadium was relaxed just an hour before kick-off and it was only with minutes to go that all the seats were taken up.
Yet with a football icon such as Diego Maradona in the crowd, and several TV news crews outside, there was no mistaking the size of the occasion.
It was perhaps that realisation which inhibited City early on. When they hit their sticky spell in March it was said the pressures of having led the table for so long had got to them.
Yet after their damaging loss to Arsenal earlier this month, when they were left trailing United by eight points, they returned to some of their free-flowing early-season best.
It appeared they were playing with nothing to lose as West Brom, Norwich and Wolves were swept aside, with the rejuvenated and restored Carlos Tevez providing the spark.
But playing for such high stakes, the tension was obvious in the early stages.
The visitors frustrated City with the likes of Ryan Giggs and Paul Scholes, winners of 12 and 10 Premier League titles respectively, using all their experience to control the pace.
United, Champions League finalists in three of the past four years, appeared in the European mode, happy to soak up the pressure and hit back on the break. City knew they had to attack, and when they did finally gain a foothold in midfield, they began to carve out a few chances.
Yaya Toure was shadowed well by Ji-sung Park early on but did break free occasionally to give United some worries.
Their goal just before half-time came after a sustained spell of pressure and exposed Chris Smalling, who was making his first start since January. He was easily outjumped by Vincent Kompany to send the City fans into raptures.
The goal transformed the atmosphere as City fans celebrated into half-time, but the nerves were evident again in the second period.
Toure grew in influence and City dictated play while United, with Wayne Rooney a frustrated and isolated figure, were disjointed.
But with the second goal not coming, there was apprehension and Mancini went defensive, sending on Nigel de Jong and Micah Richards for Carlos Tevez and David Silva.
It was a move that could have backfired. Inviting United to attack can be a recipe for disaster and there were plenty of flutters as the fourth official indicated there would be five minutes of injury time. Samir Nasri had the chance to wrap up the points in stoppage time but dawdled when he should have shot.
Yet City had done enough to claim the win that puts a first title in 44 years within their grasp.