Edin Dzeko struck inside the first minute and his side never looked back, with the Red Devils rarely mustering an effort on goal before Yaya Toure added the gloss at the death.
David Moyes' men were buoyed by good wins over Olympiakos and West Ham heading into the contest but City stung them instantly as Edin Dzeko scored after 43 seconds to register the fastest away goal at Old Trafford in Premier League history.
|VIEW FROM OLD TRAFFORD
|By Greg Stobart
The worst part of the night for the Manchester United fans here will be just how easily it was brushed aside by the team it beat to the title by 11 points last season.
City strolled to three points here to boost their title hopes - but it’s hard to even judge its credentials as title challengers against such meek opposition.
Edin Dzeko scored twice and Yaya Toure hammered home the third as City cruised to victory, with the 3,000 visiting supporters greeting each completed pass with a chant of ‘ole’ by the 60th minute.
United huffed and puffed but never looked like getting back in the game from the moment City took the lead after just 43 seconds, with David Moyes’ pre-match gameplan going up on smoke.
The only positive for United - and it’s barely a silver lining - is that Marouane Fellaini escaped a red card and subsequent suspension for an ugly elbow on Pablo Zabaleta in the first half.
United proceeded to stagger its way through the opening exchanges and rarely threatened to stop City extending its present dominance of the fixture to five wins from six league meetings, although Juan Mata might have leveled matters six minutes before the interval.
Dzeko finished precisely 11 minutes into the second half to give City breathing space, and Yaya Toure added a third in the 90th minute, the Ivorian's fourth goal in two games.
Manuel Pellegrini's team is well-placed in the title race, three points behind leaders Chelsea with two games in hand ahead of Saturday's trip to Arsenal.
It is a battle Moyes and champions United must watch from afar, having won just one of 11 matches against the six teams above them in the table this season.
Moyes selected a three-man central midfield in the hope of giving United a more robust feel, but it suffered a nightmare start.
City immediately drove into opposition territory and Rafael was forced into a superb last-ditch tackle to deny David Silva, but the ball was worked out to Samir Nasri and, in the United right back's absence, he had time to pick out a low shot which hit the inside of the post, rebounded across goal and was turned in by Dzeko.
United struggled badly to gain an early foothold and Silva should have increased its anxiety when he scuffed a ninth-minute shot against a prone Rio Ferdinand - starting in place of the suspended Nemanja Vidic at center back.
Ferdinand appeared to be struggling with the pace of the game and played Silva onside in the 18th minute, leading to Dzeko drawing an excellent stop from David De Gea.
Marouane Fellaini drove into Joe Hart's gloves, but the former Everton midfielder was perhaps fortunate to remain on the field with half an hour played after pushing his elbow into Pablo Zabaleta's face. Referee Michael Oliver decided a yellow card was sufficient.
United gradually worked its way into the half and Mata blasted over after Wayne Rooney’s 39th-minute air-shot.
Moyes sent on Shinji Kagawa for the ineffective Tom Cleverley at the start of the second half, but City resumed on the front foot.
Fernandinho could not adjust to convert Vincent Kompany's flick-on from Nasri's 52nd-minute corner and headed over.
Yet United did not heed the warning and, when Nasri produced a similar delivery four minutes later, Dzeko coolly planted a side-foot volley beyond De Gea, ultimately sealing a ninth consecutive away league game unbeaten - a far cry from City's early-season troubles on the road.
The frenetic pace of the game subsided and Hart remained severely under-worked, with the exception of a sharp 71st-minute stop that kept out an impudent flick from England colleague Danny Welbeck.
A woeful evening for United was completed when a pass from substitute James Milner deflected into Toure's path, allowing the midfielder to drive home into the bottom corner in trademark fashion.