TEN-MAN Albion's three-game winning streak was halted by a Wayne Rooney double.
The Baggies started well enough but the champions gradually got on top and Rooney deservedly fired them ahead on 36.
The visitors, who had lost just one of their previous six away games, had strong claims for a penalty turned down on 53 when Peter Odemwingie was tripped by United skipper Patrice Evra in the box.
But they faced a real uphill struggle when the otherwise rock-solid Jonas Olsson collected the second of two yellow cards in the space of 17 minutes on 66.
And United made their man advantage count five minutes later when Rooney fired them two-up from the spot after makeshift right-back Keith Andrews had fouled Ashley Young.
The Baggies failed to hit the heights of their previous three games but will take considerable solace from the fact they didn't get overrun, especially while playing with ten men.
Yesterday's results saw Albion slip a couple of places to 12th, but they still headed to Old Trafford safe in the knowledge their 13-point cushion on the drop-zone remained intact.
Second-placed United, of course, had altogether different matters on their mind, as they targeted a victory to put pressure on neighbours City, who they trailed by two points.
Roy Hodgson made two enforced changes from the Chelsea victory, with Gabriel Tamas and skipper Chris Brunt replacing injured duo Steven Reid (ankle) and Jerome Thomas (hamstring).
Paul Scharner and Somen Tchoyi were both recalled to the bench, where they were joined for the first time this season by former United Academy keeper Luke Daniels, who was in for chicken pox-victim Marton Fulop.
Sir Alex Ferguson made one change from United's last league outing, with Javier Hernandez replacing Nani.
Backed by a vocal 1,800-strong away support, Albion soon settled into their 'big stage' surroundings and after defending an early United corner, the Baggies almost snatched a fifth-minute lead.
Brunt brilliantly dispossessed Phil Jones and James Morrison fed Liam Ridgewell down the left, whose inviting centre was poked wide at the near post by Andrews at full stretch.
Albion had not travelled to the Theatre of Dreams to defend and, in the ninth minute, both Andrews and Gareth McAuley came within a whisker of connecting with Olsson's centre, before Morrison shot over from the edge of the area.
However, ex-United man Ben Foster was soon forced into his first save. Danny Welbeck rode several Albion challenges before teeing up Young in the box and the former Villa ace's acute drive was pushed away by the Baggies No.1 at his near post.
Rooney was the next United player to try his luck, but his 25-yard effort flew well wide from Hernandez's lay-off.
After a brief lull in the action, Albion won a free-kick 30 yards out and David De Gea had to pull off a fine save low to his left to keep out Andrews' fierce drive. From Morrison's ensuing corner. Ridgewell won the penalty-area aerial contest but directed his flick header wide.
The Baggies looked comfortable as the half ticked past its midpoint, but they were tested by two quick-fire United corners in the 26th minute. After dealing with the first, Welbeck headed Young's second delivery over from eight yards.
Albion had a real scare a minute later. Young exchanged an exquisite one-two with Welbeck before slipping the ball to the side of the onrushing Foster in the hope of giving Rooney a simple tap-in - but Olsson intercepted to deny the England striker a certain goal.
The visitors were now struggling to get out of their own half and they were relieved to see Rooney direct an awkward volley harmlessly at Foster from Young's angled ball into the box.
But United's pressure paid off on 36 when they got their noses in front.
Hernandez cut inside Ridgewell on the right and saw his 18-yard strike, which Foster had covered, poked home by the predatory Rooney from six yards.
United immediately pushed for a second and Jones directed an awkward header into Foster's side-netting from another terrific Young centre.
On 44, it took a last-ditch interception by Olsson to deny Welbeck a clear sight of goal after a mesmerising United move involving Paul Scholes, Michael Carrick and Young.
Albion managed to make more of a fist of it during the opening stages of the second half and Odemwingie had a penalty appeal turned away on 53 when he went to ground under Evra's challenge. TV replays suggested Odemwingie had been fouled.
United surged straight upfield and went agonisingly close to securing a two-goal cushion when Hernandez collected Rooney's pass and rattled the left-hand post with a ferocious shot from just inside the box.
Moments later, Rooney directed an audacious overhead kick from Evra's centre straight into Foster's grateful arms.
The Red Devils went even closer just before the hour-mark when a harmless Scholes punt suddenly turned into a glorious throughball when the officials missed a blatant foul by Hernandez on McAuley on the edge of the Albion box. The ball zipped past the wrong-footed Foster, only for Welbeck to side-foot wide of the open net from a tight angle.
Back came Albion and Tamas smashed an ambitious 35-yarder straight at De Gea.
After Foster had easily gathered Scholes' 25-yard drive, Albion had a mountain to climb when Olsson was dismissed for a second bookable offence in the 66th minute. The mistimed tackle on Hernandez had been preceded by a similar misdemeanour on Jones 17 minutes earlier.
United immediately tried to turn the screw and after a goalmouth melee, Foster pulled off a stunning save to keep out Young's 15-yarder, which took a wicked deflection off Tamas, who had moved into central defence following Olsson's dismissal.
But it looked game over when United made it 2-0 on 71.
Makeshift right-back Andrews pulled down Young inside the box and Rooney made no mistake from the penalty spot, slipping the ball inside the left-hand post.
United were in complete control and Hernandez was just wide with a side-footed shot from the edge of the area.
Albion had not given up the ghost, though, and De Gea had to make a fine save to keep out sub Shane Long's near-post drive after a fine breakaway move spearheaded by Youssouf Mulumbu.
United had another minor scare on 88 when Evra almost headed Tamas' centre into his own net.
UNITED (4-4-2): De Gea; Jones, Ferdinand, Evans, Evra; Welbeck (Cleverley 77), Carrick, Scholes (Pogba 72), Young; Rooney, Hernandez. Subs not used: Amos (gk), Giggs, Park, Rafael, Berbatov.
ALBION (4-5-1): Foster; Tamas, McAuley, Olsson, Ridgewell; Odemwingie, Mulumbu, Morrison, Andrews (Scharner 71), Brunt (Shorey 71); Fortune (Long 77). Subs not used: L Daniels (gk), Cox, Tchoyi, Dorrans.
GOALS: UNITED - Rooney (36 & 71 pen).
BOOKINGS: ALBION - Olsson (foul 49).
SENDING-OFF: ALBION - Olsson (foul, second bookable offence, 66).
REFEREE: L Probert (Gloucestershire).
ALBION STARMAN: Ben Foster.