At the same time they killed off United's dream of repeating their Champions League, Premier League and FA Cup treble of 1999 after Yaya Toure struck a 53rd minute winner at Wembley.
Favourites United began positively and dominated the opening half-hour but they were then out-muscled by a City team famously dismissed by manager Sir Alex Ferguson as "noisy neighbours".
United's misery was complete when midfielder Paul Scholes was red-carded 18 minutes from time for a lunge on City defender Pablo Zabaleta and their players ended up pushing and shoving opponents and a TV cameraman in a melee after the final whistle.
City's victory, totally deserved after they recovered from a sluggish start, gives them the chance of a first major hounour for 35 years to repay the super-rich sheikh who bought the club in 2008 and has since invested a fortune in players.
"I'm very happy for our supporters because they have waited a long time for a day like this," he said. "But for us it's very important that we don't forget we have another game. We only won a semi-final today."
When he bought the club two-and-a-half-years ago Sheikh Mansour vowed to turn City into one of the world's top clubs, but they have yet to qualify for the Champions League or win a trophy.
A poor clearance from Edwin van der Sar and a sloppy pass from Michael Carrick allowed Ivory Coast international Toure to escape from the United defence and he fired home through the united keeper's legs for what proved to be the winner.
"The first 15 minutes after halftime cost us the game," said Ferguson. "Slack moments. Edwin had a bad kick out and Michael Carrick couldn't hold it and it was a goal.
"From then on they were defending apart from a couple of counter-attacks. It's disappointing as we should have been ahead in the first half as we were the better team.
"The chance Dimitar missed there was a great save by the keeper but the second chance from under the bar if he'd have scored there I had a feeling whoever scored first would win the match," he added on MUTV.
