Kieron Dyer's cross-shot from the left was palmed out by Anders Lindegaard and Jamie Mackie found the roof of the net from close range.
Back on trackAfter losing at Norwich City and Galatasaray in their last two outings, United were staring at the prospect of a third consecutive defeat, but three goals in the space of eight minutes saw them roar into the lead.
Jonny Evans headed in the equaliser from close range in the 64th minute and four minutes later, Darren Fletcher marked his first league appearance in over a year by scoring with a thumping header from Wayne Rooney's corner.
Substitute Javier Hernandez then beat QPR goalkeeper Julio Cesar from Anderson's pass in the 72nd minute to kill the game off and lift Sir Alex Ferguson's men two points above Manchester City at the Premier League summit.
City are not in action until Sunday, when they visit fourth-place Chelsea and their new interim coach, Rafael Benitez.
Everton were prevented from closing to within a point of fourth-place Chelsea as Sebastien Bassong netted a last-minute equaliser to earn Norwich a 1-1 draw at Goodison Park.
Earlier, West Bromwich Albion continued their superb start to the season with a 4-2 win at Sunderland that took them up to third place.