Darren Fletcher's first-half header ensured Manchester United ended Chelsea's 40-match league unbeaten run.
Fletcher appeared to be trying to head the ball back across the goal, but his effort looped inside the far post.
Early in the second half Ruud van Nistelrooy should have made the game safe but he blazed his shot over.
His miss meant United had to live on their nerves as Chelsea desperately sought the equaliser and Edwin van der Sar bravely denied Frank Lampard.
The win saw United climb six places to third, leaving them 10 points behind Chelsea, with a game in hand on the league leaders.
On the 19th anniversary of his appointment as United manager the win provided Sir Alex Ferguson with something to celebrate in what has been a miserable week.
In that time United have been humbled by Middlesbrough, have lost to Lille in the Champions League and Roy Keane has questioned the calibre of some of the club's recent signings.
Fletcher was one of the players singled out by Keane for criticism but, with the Irishman watching from the stands, the young Scot produced a storming performance.
It was his cross that provided that gilt-edged chance for Van Nistelrooy in the second half when the Dutchman thrashed his shot over the bar.
Late on Wayne Rooney might have snatched a second goal for United, but his goalbound shot was deflected for a corner by Paulo Ferreira.
This was only the second Premiership defeat Chelsea have suffered under Mourinho since he took over at Stamford Bridge in the summer of 2004.
Following their midweek Champions League defeat to Real Betis it is the first time Chelsea have lost two league games in a row under their Portuguese manager.
Ten minutes into the second half Mourinho replaced Michael Essien with Eidur Gudjohnsen to try to provide Didier Drogba more support.
Gudjohnsen's introduction prompted Chelsea to lay siege to the United goal for the rest of the half.
The Icelandic striker quickly made an impact, passing to Damien Duff who, with the goal at his mercy, miscued his shot.
Van der Sar's courageous block to thwart Lampard - the Dutchman threw himself at the Chelsea midfielder's feet - was the save of the game.
If the game's closing stages were frenetic, then its opening exchanges were equally frantic.
Drogba tried to lob Van der Sar but the Chelsea striker was unable to get enough power in his shot.
Asier Del Horno also had a glimpse of the United goal, only to volley wide a Joe Cole free-kick.
In the opening half-hour Ronaldo had a succession of opportunities to cross from the left, each time failing to provide a worthwhile delivery.
But he finally got his angles correct to pick out Fletcher at the far post.
Fletcher headed the ball back across goal and his flighted effort took on an altogether more deadly trajectory floating over John Terry and landing in the back of the net.
Man Utd: Van der Sar, O'Shea, Ferdinand, Brown, Silvestre, Fletcher, Smith, Scholes, Rooney, van Nistelrooy (Park 82), Ronaldo.
Subs Not Used: Howard, Richardson, Bardsley, Rossi.
Booked: Smith, Ronaldo, Fletcher.
Goals: Fletcher 31.
Chelsea: Cech, Paulo Ferreira, Gallas, Terry, Del Horno (Carlton Cole 78), Essien (Gudjohnsen 55), Makelele, Lampard, Joe Cole (Wright-Phillips 74), Drogba, Duff.
Subs Not Used: Ricardo Carvalho, Cudicini.
Booked: Drogba, Paulo Ferreira, Gallas, Makelele.
Ref: G Poll (Hertfordshire).