Goals from Joleon Lescott and Edin Dzeko had put the Blues in command against the Reds, but Chris Smalling and Nani had levelled the scores before the hour-mark as both sides served up a Sunday afternoon treat for the watching millions around the world.
But it was an uncharacteristic mistake from skipper Vincent Kompany let in Nani deep into injury time as a penalty shoot-out had loomed.
Starting the new season with a Manchester derby at Wembley is a new experience, but one every Blue could get used to, though obviously with a different outcome next time around.
City’s pre-season form has been impressive and with the Premier League start against Swansea just a week away, this was the final chance to fine-tune the starting XI as well as perhaps laying down a marker for the upcoming campaign.
With Sergio Aguero starting on the bench, the preferred strike duo was Edin Dzeko and Mario Balotelli, who linked to great effect against Inter Milan recently. James Milner got the nod ahead of Adam Johnson and Gareth Barry while Aleks Kolarov started ahead of Gael Clichy.
With the early morning sunshine giving way to drizzle by kick-off, the greasy pitch was initially handled better by United whose passing was crisper and more accurate in the opening stages, leading to a couple of goal-mouth scrambles that could have resulted in a goal.
The Blues, just as they had done in the FA Cup semi-final against the Reds, gradually played their way into the game and as the tackles flew in from either side and yellow cards began to flow, it soon underlined that this was far more than just a showpiece curtain-raiser and seven minutes before the break, the match finally had its first goal.
Milner’s dashing run was halted by Patrice Evra on the edge of the box and from the resulting free-kick Silva’s pinpoint cross found Lescott who glanced the ball past De Gea to send the City fans wild.
While the Poznan was still shaking the Wembley foundations, the Blues went 2-0 up as they began to pick United apart at will. This time it was Edin Dzeko - the first Bosnian to play at Wembley – who once again sent the Blue Army into raptures when he let fly a 30-yard thunderbolt on the stroke of half-time.
Having established some breathing space, it was crucial City keep United at bay for at least the opening 20 minutes of the second half, but instead slack marking allowed Chris Smalling the chance to guide Ashley Young’s cross past Joe Hart with just 52 minutes on the clock.
There was worse to follow six minutes later as United drew level when Nani worked his way through to chip the ball over Hart from close range with the Blues looking a little leg-weary with still 30 minutes to play.
As United swarmed forward, City managed to repel a series of attacks that threatened to engulf Mancini’s side at one point but it was Nani who ensured the first piece of silverware this season went to Old Trafford and not the Etihad Stadium.