Goals from Bernt Haas and substitute Scott Dobie - with his first touch - settled the tie at The Hawthorns while Jason Koumas also had a penalty brilliantly saved by Roy Carroll.
The opening goal arrived via the first menacing attack of the encounter as the influential Koumas fed Neil Clement down the left flank. The full back's cross was headed on by Rob Hulse and Swiss defender Haas unleashed a stunning volley beyond Carroll.
The Baggies were putting their attacking aerial presence to good use during the early exchanges as United struggled to gain a foothold in the game.
West Brom should have doubled their advantage midway through the half when Koumas was fouled by Danny Pugh but the Welsh international's penalty was superbly saved by Carroll.
Koumas was the spark behind all of the hosts' attacking threat and carved out another opportunity - this time for Daniele Dichio but the forward's header was comfortably gathered by Carroll.
It was only in the closing ten minutes of the first period that the visitors stepped up a gear with Nicky Butt's volley under pressure easily saved by Russell Hoult.
Stand-in captain Butt wasted a glorious chance to equalise on the stroke of half time when he was picked out by David Bellion unmarked on the penalty spot but the England midfielder's header was directed straight at the Baggies keeper.
Dichio is not always renowned for using his height to great effect but three minutes after the interval he headed against the crossbar with Carroll beaten.
United responded immediately with Cristiano Ronaldo feeding Kleberson and the Brazilian's curling shot was smartly pushed away by Hoult.
The Baggies nicked the crucial second goal on 56 minutes when Clement's cross from the wing enabled Dobie to steer a low header past Carroll.
United huffed and puffed in search of a way back into the match but their tendency to be caught offside proved costly as the home side remained a constant threat on the counter attack.
In the final minute, The Red Devils had a chance to snare a consolation after Darren Fletcher pulled the ball back for Kieran Richardson but the young winger's shot was headed away.
