Juan Mata scored a late, late goal to down the league champions and put the Blues in the box seat to qualify for next season's Champions League.
The mercurial Spaniard arrowed a low left-footer across the path of goalkeeper Anders Lindegaard from just inside the United penalty area, three minutes from time to send the Chelsea fans into delirium.
The west Londoners are now in third place in the league, a point ahead of Arsenal with a game in hand, and more crucially three points in front of fifth place Spurs.
The two meet at Stanford Bridge on Wednesday night in what could very well be a final Champions League qualification encounter.
While final places are not determined in midweek, the winners will almost certainly be favourites to take at least fourth.
As for the match itself, it was a turgid affair with an end of season air to it.
United showed very little in the way of attacking prowess, while Chelsea were determined not to lose and hope for a slice of luck.
That they got.
Chelsea startle off the encounter well and on six minutes Cesar Azpilicueta's rightly line cross caused Inited defence to panic. Unfortunately for the Blues Demba Ba could not take control of the ball quick enough from close range and the half-chance went begging.
Two injures lat and Ashly Cole was beaten by the paste of Antonio Valencia and his byline cross was low. Petr Cech fumbled the pick-up but David Luiz was on hand to clear.
Nine minutes in and the Chelsea faithful in response to United's fans insulting Rafa Benitez decided, instead of a stinging riposte and support for their team's boss, to chant the name of the man they want to replace him - Jose Mourinho.
Oscar, was very unlucky on 14 minutes not to open the scoring, when after a quick counter-attack, the Brazil midfielder raced virtually the length of the Old Trafford pitch before letting fly with a shot with the outside of his right boot. The ball arrowed towards goal from the edge of the United penalty area. Goalkeeper Anders Lindegaard got his fingertips to it, but the ball tricked onto his left upright and back into his gloves to safety.
Victor Moses tried a 20 -yarder a few minutes later but his wayward effort ended up in row Z of the Stretford End stand.
United counter themselves on 18 minutes with a similar result from Robin Van Persie whose left footer was well wide.
Anderson was inches away from tucking home from close range after Phil Jones's excellent right wing cross.
Frank Lampard, desperate to equal the club goal-scoring record of 202 set by Bobby Tambling tried his trademark daisy-cutter on 29 minutes but the 25-yard effort, while on target, was not powerful enough to trouble Lindegaard.
Out of nothing and United should have taken the lead, three minutes before the break. A brilliant defence-splitting ball by Ryan Giggs sent Van Persie free on the left. The former
Arsenal star touched the ball on, past Cech, but it went inches past the Chelsea goalkeeper's right post.
A half chance by Oscar and Van Persie, the later a header straight to Cech signalled the end of a mostly non-event of a half.
The Blues had the first effort of the second half, four minutes in, a classic near-post half volley by Ba after excellent set-up play by Moses. The ball was deflected for a corner, much to the relief of the United fans.
Lampard arrived late into the United penalty area in characteristic fashion to a Mata flick but his volley was overhit and wide of goal.
Sir Alex Ferguson decided, with 20 minutes to go, to make a couple of changes, Wayne Rooney and Alexander Buttner on for Tom Cleverley and Anderson.
Rooney almost grabbed a goal with a glancing header three minutes later.
But the 75,500 groaned as the effort zipped over Cech's crossbar.
Interim manager Benitez introduced Fernando Torres for Moses with 15 minutes remaining.
Had Mata being a couple of inches taller, he would have being able to head home from six yards out after Lampard clipped in the perfect pass.
Ivanovic almost scored in his own net after Rafael's shot took a deflection off the Serbia defender's leg as United sought the goal to justify their status as Premier League champions.
The visitors had a chance with a Luiz free kick on 83 minutes but his effort failed to trouble the United goal.
But with three minutes to go, Chelsea took the lead.
Rooney thought he had been fouled just outside the Chelsea penalty area, but referee
Howard Webb dismissed the appeal. Chelsea raced up field and after Ramires passed to Oscar he sent the ball wide to the left of the United area to Mata. The Spain midfielder arrowed a low left footer across goal. The ball took a slight deflection off Nemanja Vidic's leg and past Lindegaard into the goalkeeper's left hand side of the net.
A minute later and a frustrated Rafael Da Silva hacked out at Luiz by his corner flag. The Brazil full-back was rightly issued a red card by Webb.
It was a dramatic end to a match that up until the 87th minute looked like it was petering out to a hopeless draw.
Mata thought differently.
Teams: Chelsea - Cech, Azpilicueta, Ivanovic, Luiz, Cole, Ramires, Lampard, Oscar, Mata, Moses,Ba
Subs: Turnbull, Ferreira, Cahill, Terry, Ake, Benayoun, Torres
Manchester United: Lindegaard, Rafael, Evans, Vidic, Evra, Jones, Anderson, Cleverley, Valencia, RvP, Giggs.
Subs: De Gea, Ferdinand, Rooney, Hernandez, Scholes, Kagawa, Buttner
Referee: Howard Webb