Goals from Phil Jones, Wayne Rooney and Danny Welbeck gave United back-to-back league wins for the first time since December, following their 2-0 success at Crystal Palace last month.
Victory took the defending champions to within nine points of fourth-place Manchester City, although they have played two more games, while Pepe Mel's West Brom remain a point above the relegation zone in 17th place.
"I thought our forward play was very good," United manager David Moyes told BT Sport.
United's last game at The Hawthorns had been Alex Ferguson's last as manager and his successor, Moyes, was looking for a reaction to his side's shock 2-0 loss to Olympiakos in the Champions League.
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In reply, Claudio Yacob and Victor Anichebe each went close for West Brom, but Foster was fortunate to escape censure after he miscalculated an attempt to control a lofted pass from Mata outside his area and the ball struck him on the hand.
West Brom forced United onto the back foot early in the second half, with Zoltan Gera testing David De Gea at his near post and Mata hooking away a low cross from Anichebe.
Gera worked De Gea again as West Brom probed for a path back into the game, but Welbeck killed off the home fans' hopes of a comeback in the 82nd minute when he ran onto Rooney's through ball to curl a shot past Foster.
Later on Saturday, Chelsea will look to extend their lead at the top of the table by beating Tottenham Hotspur at Stamford Bridge.
A win would lift Chelsea seven points above Liverpool and Arsenal, who both have a game in hand, and nine points clear of fourth-place Manchester City, who will have three games in hand.