It was business as usual for United who drew 1-1 at Blackburn Rovers to earn the point they needed to win the Premier League but City's 1-0 victory over Stoke City secured their first trophy for 35 years.
Fans of big-spending City will hope the triumph begins an era of domination similar to the one arch-rivals United have enjoyed since their FA Cup succes in 1990 sparked a run of 25 major trophies in 21 years.
"We won the FA Cup. It is an important trophy," City manager Roberto Mancini said. "We need to improve more to do another step but it was important to start to win because when you start to win, afterwards everything will be easier."
United manager Sir Alex Ferguson was delighted his side had become the most successful team in English football, finally knocking arch-rivals Liverpool "off their perch" as he promised to do at the start of his managerial reign at Old Trafford.
"It's not so much passing Liverpool," Ferguson said. "That thing about knocking Liverpool off their perch, I don't think I actually said it, but it's more important that United are the best team in the country in terms of winning titles.
"Same with the FA Cup. We have won it more times than everyone (11) and now we have won the Premier League more times than anyone."
DUG DEEP
United fell behind against Blackburn when Brett Emerton scored for the hosts midway through the first half at Ewood Park but, as so often this season, Ferguson's team dug deep to get what they needed.
Mexican striker Javier Hernandez was fouled by Blackburn goalkeeper Paul Robinson as he chased a long ball and, after a consultation with his linesman, the referee awarded a penalty.
Wayne Rooney stepped up to convert the spot-kick but he was not feeling as calm as he looked.
"It was terrifying," said the England striker who had stunned United by asking for a transfer in October but changed his mind and returned to form in time to play a big part in their title win.
City dominated the match against Stoke at Wembley but failed to find the net until the 74th minute when midfielder Yaya Toure blasted the ball home from 12 metres.
"We bought him for this," Mancini said. "He is a fantastic player. He scored in the semi-final (against Manchester United) and the final. All season he has played very well."
United's thoughts will now turn to the Champions League Final against Barcelona on May 28 at Wembley while City will be pondering playing in next season's competition.
"We have got to the Champions League, that was our first target, and we won the FA Cup," Mancini said. "It is very important for this club."
