ALBION were sentenced to a cruel defeat against the champions by Steven Reid's 81st-minute own goal.
The Ireland international knew little about United's winner, after Ashley Young's low cross squirmed off his foot and inside the far post.
It was rough on the Baggies, who had comfortably contained United during the second half.
The visitors had taken the game to Albion and took the lead through Wayne Rooney's fine 13th-minute finish.
But Shane Long's debut strike eight minutes before the break restored parity as Roy Hodgson's side finished the half on top.
Albion may have begun the new campaign in defeat but Hodgson will take plenty of positives from his team's disciplined display.
New boy Long was thrown straight in at the deep end as, with Peter Odemwingie ruled out by an ankle problem, he was handed an immediate start.
Jerome Thomas also missed out through injury but Reid recovered in time from a calf problem.
New loan signing Ben Foster made his competitive Baggies bow against his former employers while United's new buys David De Gea and Young were granted league debuts.
There was a touching moment during the pre-match warm-up when the Albion players wore t-shirts bearing a 'get well soon' message to kitman of 50 years, Dave Matthews, who is recovering from a serious illness.
Both sets of players also wore black armbands during the match in memory of the three men - Haroon Jahan, Shazad Ali and Abdul Musavir - who tragically died on nearby Dudley Road during this week's disorder.
United were determined to stamp their authority on the game from the first whistle and the hosts had to weather concerted pressure during the opening stages, including a barrage of set-pieces.
But the Baggies survived and went on to record the first shot of the game in the eighth minute when Paul Scharner saw an audacious 40-yard drive fizz just wide of the left-hand post.
Somen Tchoyi was looking lively and had a close-range effort blocked by Nemanja Vidic after United struggled to clear a Chris Brunt corner.
But the 13th minute proved unlucky for Albion - as United took the lead.
A great exchange between Rooney and Young created space for the former just inside the Baggies box and he fired unerringly past Foster and into the far corner.
Two minutes later, Rooney threatened again when he was allowed to advance forward before firing wide of the far post from 20 yards.
Albion gradually regrouped and began to push United back - but the visitors should have been two-up on 22.
Danny Welbeck and Rooney combined brilliantly to present Nani with a clear sight of goal from ten yards, only for the Portuguese international to fire well over.
Albion went close themselves three minutes later when Brunt's in-swinging free-kick fell nicely for Reid, who swivelled and saw his 15-yard effort deflected wide by Fabio.
Moments later, Long exchanged a neat one-two with James Morrison before drilling past the left-hand post from 25 yards.
But United were living up to their billing and seemingly caused problems every time they entered the Albion half.
On 29, Nani again should have done better when he volleyed over from 12 yards after Foster failed to get enough purchase on his punch from Young's flag-kick.
But Albion started creating opportunities of their own and only a wonderful one-handed save by De Gea denied Scharner's stinging 15-yard drive after the midfielder had combined brilliantly with Long.
Just two minutes later, it was 1-1.
Tchoyi showed terrific strength to keep hold of the ball under pressure from Fabio and feed Brunt, who immediately found Long just inside the box.
The new No.9 made enough room to squeeze a right-footed shot past nearest-defender Chris Smalling and it crept under De Gea's arms before nestling in the far corner of the net.
For the first time, a sense of uncertainty had crept into United's play and Brunt sent an awkward 12-yard volley high into the Smethwick End after the men in red failed to clear their lines.
More questionable defending saw Albion go close again on the stroke of half-time.
De Gea could only palm Tchoyi's awkward cross to Scharner, who fired across the face of goal from a tight angle.
It was even-stevens during the early exchanges of the second half and it wasn't until nine minutes after the interval that either goal was threatened, with Rooney flicking a header that Foster had covered wide from Young's in-swinging free-kick.
Albion had done a much better job of containing United since the interval, which prompted Sir Alex Ferguson to replace Welbeck with Dimitar Berbatov on 65.
United were struggling to find a way through and Young resorted to a cross-cum-shot on 69, which arced just wide of the far angle of post and bar.
A minute later, Nani was well off target with a header from the overlapping Fabio's left-wing cross.
By the 75th minute, United were playing with a different centre-back pairing than the one they started with when Rio Ferdinand hobbled off - some 23 minutes after the injured Vidic had.
Albion had to be patient for chances and De Gea endured more nervy moments in the 79th minute when he struggled to deal with a couple of Albion set-pieces.
But United finally shattered the Baggies' resistance nine minutes from time - in fortuitous circumstances.
Young cut in from the left and saw his low cross clip Gabriel Tamas and then Reid before screwing past the helpless Foster and inside the far post.
Albion plugged away for an equaliser during the dying minutes, with Tchoyi shooting over from an acute angle and Brunt testing De Gea with a dipping 20-yard drive.
ALBION (4-4-1-1): Foster; Reid (Jara Reyes 85), Tamas, Olsson, Shorey; Brunt, Scharner, Mulumbu, Morrison; Tchoyi, Long (Cox 87). Subs not used: Fulop (gk), Dorrans, Dawson, Thorne, Fortune.
UNITED (4-4-2): De Gea; Smalling, Vidic (Evans 52), Ferdinand (Jones 75), Fabio; Nani, Cleverley, Anderson, Young; Welbeck (Berbatov 65), Rooney. Subs not used: Lindegaard (gk), Giggs, Park, Carrick.
GOALS: ALBION - Long (37). UNITED - Rooney (13), Reid (81 own goal)
BOOKINGS: ALBION - Reid (foul 38), Scharner (foul 86), Cox (simulation 90+2). UNITED - Young (foul 56), Evans (dissent 78).
REFEREE: M Jones (Chester).
ALBION STARMAN: Somen Tchoyi.