A goal from Yaya Toure seven minutes into the second half decided a typically tight derby in which Paul Scholes was sent off for recklessly raking of his studs down the thigh of Pablo Zabaleta.
Now the real fun begins.
For not only has this famous success given the Blues a shot at a first domestic final since 1981 but it is the perfect tonic for the Premier League run-in and proved once and for all that the Blues can win without Carlos Tevez.
Six league games and the FA Cup final now follow in rapid succession and with the right results these City players will be making history come the end of the season.
For the next few weeks for the squad at least the FA Cup will go on the backburner.
Champions League qualification will be the aim and a new even bigger stage in which to stand toe to toe with United
For that is what Mancini’s men did in this touring London production of a Mancunian neighbourly spat.
Cowed in the face of the Red tide flowing their way at Anfield the previous Monday night, this was a very different City side though they took half an hour to get going as the 4-2-3-1 systems cancelled one another out.
In that time with Scholes and Carrick pulling the strings, Berbatov missed an absolute sitter from three yards out and forced Joe Hart into the kind of brave instinct save that makes him the nation’s number one.
Vidic also went close with a header before the Blues decided to defend less deeply and keep the ball a little better.
The result was Gareth Barry hitting the side-netting, Balotelli stinging Van der Sar’s knuckles from 35-yards, Joleon Lescott volleying over the bar with his ‘wrong’ foot and captain Vincent Kompany curling a twenty-yarder just wide.
Ironically given their contribution to early United dominance it was Carrick and Scholes who engineered their own team’s downfall
Carrick was negligent and careless giving the ball away to Toure who burst past Vidic before sliding the ball confidently beyond Van der Sar. It was a first goal on someone else’s pitch since February.
In an instant all the heartache endured against United in recent luckless defeats seemed trifling. City’s support, buoyant and boisterous all afternoon went delirious and so did Paul Scholes.
The only Mancunian on the pitch who had chided City as ‘not rivals’ in the build was sent off and as he trudged off so did the Reds hopes of levelling matters – though there was still time for one more superb save by Hart from Nani.
Five minutes of added time simply added to the pleasure rather than the agony for Blues who will be back for a May 14 showdown.
It promises to be a watershed moment for Mancini. City’s talismanic Tevez was missing but there was no lack of energy, no shortage of vim and vigour and no shirking the questions posed in the Wembley sunshine.
The new stadium, playing host to the Blues for the first time, was a fermenting cauldron from start to finish.
Tribal it may have been but the visceral atmosphere was all that is so memorable about the FA Cup and English football. Long may it last.
Mancini, in the absence of Tevez adorned the muscled, gladiatorial arm of Kompany with the captain’s armband, chose an attacking side with Adam Johnson in from the start and Mario Balotelli the spearhead.
The players and coaching staff danced the Poznan at the end of the game let’s hope they are doing it again next month.