It was United's first home defeat in the Premiership for 13 months and there could be no arguments about a score which Fulham thoroughly deserved.
They harried, outchased and outclassed a United side which lacked even the more rudimentary aspects of the game let along imagination.
Fulham's livewire forwards Steed Malbranque, Luis Boa Morte and Louis Saha ran rings around a leaden-footed United defence.
The Londoners defence coped comfortably against out of sorts United forwards Ruud van Nistelrooy, Diego Forlan and Ryan Giggs while Fulham held all the trump cards in midfield where there was little United sparkle.
United manager Alex Ferguson banished to the stands following his touchline ban, would have watched in horror as United's defence was torn apart in the third minute.
United centre-back Mikael Silvestre gave the ball away to Malbranque on the right and he beat Nicky Butt on the edge of the box before crossing for Lee Clark to score with a clever left-foot shot at the near post.
Fulham should have extended their lead a quarter of an hour later when Mark Pembridge connected with Boa Morte's right-wing cross on the edge of the United box but his shot crashed back off the underside of the bar.
United keeper Tim Howard made fine saves from Malbranque and Saha while the Red Devils were thrown a lifeline almost two minutes into first-half injury time.
For once a United move went right as Van Nistelrooy laid the ball to Giggs and his superb throughball sent Forlan racing clear down the right with the Uruguayan tucking his right-foot shot beyond Fulham goalkeeper Edwin Van der Sar and into the far corner of the net from the edge of the penalty area.
Ferguson must have expected his goal to act as the springboard for a second-half revival, but United slipped back into terminal lethargy.
The awful turned into the bizarre four minutes after the break.
Quinton Fortune on at the start of the half, substitute for the injured Silvestre was booked by Mike Riley for coming on without the referee being informed.
United plunged back into the depths after 66 minutes when Fulham regained the lead.
Rio Ferdinand failed to cut out Boa Morte's cross from the left, Saha nodded the ball on and Malbranque scored with a crisp low right-foot shot from the edge of the penalty area.
Howard saved bravely at Malbranque's feet in the 78th minute, but Fulham made it 3-1 seconds later.
The influential Malbranque found substitute Junichi Inamoto on the left corner of the United penalty area and he swept his left-foot drive beyond Howard and into the far corner of the net.
Van der Sar saved well from United sub David Bellion at point-blank range two minutes from time, but there was to be no escape for sorry United.
Man Utd: Howard, Gary Neville, Ferdinand, Silvestre (Fortune 45), O'Shea, Ronaldo (Scholes 69), Djemba-Djemba (Bellion 80), Butt, Giggs, Forlan, van Nistelrooy. Subs Not Used: Carroll, Fletcher.
Fulham: Van der Sar, Volz, Goma, Knight, Bonnissel (Djetou 74), Malbranque, Pembridge (Inamoto 32), Clark, Legwinski, Boa Morte (Hayles 86), Saha. Subs Not Used: Crossley, Melville.
Attendance: 67,727
Referee: M Riley (W Yorkshire)