Jonathan Fortune and Richard Cresswell struck before the break with skipper Chris Morgan finishing the Royals off in the 80th minute.
Reading, fresh from a 1-0 FA Cup victory over Burnley, escaped in the opening minutes after midfielder Stephen Quinn found Cresswell but the ex-Stoke striker squandered the chance, heading over from eight yards.
The Championship strugglers were in trouble again, winger Jamie Ward slipping inside to drill in a fierce left-foot shot that was tipped round the post by Adam Federici.
But the Reading keeper was helpless in the 12th minute, James Harper threading a pass to Fortune who coolly brought the ball down before volleying past Federici from six yards.
Reading were handed a chance to level matters in the 40th minute when United centre-back Morgan fouled Shane Long in the area. Gylfi Sigurdsson stepped up to take the spot-kick only for Mark Bunn to pull off a spectacular stop diving at the foot of the post to tip the ball away.
United turned the screw and surged further ahead two minutes later, full-back Kyle Walker pumping over a teasing cross that Federici could only palm into Crseswell's path and he headed in from close range.
Reading stepped up the pace after the break. Jem Karacan tried a long-range strike that flew over the target and Sigurdsson was foiled by Bunn.
But United always had the upper hand, Cresswell almost grabbed a second, his strike from 12 yards looping inches over the top before Jordan Stewart, on for Andy Taylor, lashed in a low shot from the edge of the area that flashed inches wide of the post.
Morgan killed off Reading ten minutes from time when he steered Ward's cross in at the far post.