The Royals have still not won a league game under new manager Brendan Rodgers, scoring just once in four matches.
Reading went ahead in the 13th minute through defender Matt Mills, signed from Doncaster for £2million.
Young Icelander Gylfi Sigurdsson, making his first league start for the Royals, floated a free-kick high into the penalty area where Mills out-jumped two defenders to head firmly past United keeper Mark Bunn.
Reading were dominating at this stage, forcing a succession of corners and Sheffield were struggling to avoid going further behind.
At one point United skipper Chris Morgan was put under such pressure that he cynically fouled Ryan Bertrand, who was in full flight towards goal, and was shown the yellow card.
Surprisingly it was Sheffield who scored next just two minutes before the break.
An apparent shot by Ched Evans went right across the face of goal and Stephen Quinn ran in unchallenged at the far post for the simplest of tap-ins past the stranded Reading keeper Adam Federici.
A blunder by Federici allowed United's Jamie Ward to put the visitors in front with just under half an hour to go.
Ward's long-range shot seemed to be covered by the Australian international but he allowed it to slip from his grasp and into the net.
United substitute David Cotterill was then shown the yellow card for the latest in a long series of fouls on Bertrand.
There was a scare for United when Andy Taylor almost put the ball into his own net but it bounced wide of the post.
Just as the tannoy announced four minutes of added time at the end of the game, United made the points safe.
An Alex Pearce clearing header fell to substitute Cotterill, who looked up at goal and calmly shot past the diving Federici.