James Harper produced a spectacular strike to leave Neil Warnock's play-off specialists still looking for their first win.
The 23-year-old midfielder snatched his second goal of the season three minutes before the interval after the Bramall Lane outfit had wasted a hat-trick of chances.
Former Fulham team mates Barry Hayles and Jon Harley were the villains, with Hayles still waiting to get off the mark since his move from London.
Midfielder Andy Liddell, signed from Wigan in the summer, picked out another new boy Harley but the ex-Fulham man miscued his left-foot strike from ten yards.
Hayles threw away a simple chance in the 17th minute after full-back Leigh Bromby's long throw from the right caused havoc in the Reading defence.
Andy Gray stretched and missed as the ball fell nicely at Hayles' feet, but from six yards he blundered, pulling his shot wide of the far post.
Harley almost made amends minutes later, unselfishly threading the ball to Hayles but Marcus Hahnemann was quickly off his line to scoop up the ball.
United tightened the screw as Harley was handed another opportunity. Hayles' fierce drive struck a defender and spun towards the United midfielder who slammed his left-foot strike into the side netting from 12 yards.
Reading survived the barrage and almost snatched the lead in the 30th minute after catching United on the break. Nicky Forster, shadowed by Phil Jagielka, cleverly turned before losing control of the ball. Dave Kitson steamed in to drill in a low strike from the edge of the area, spectacularly turned round the post by Paddy Kenny.
Republic of Ireland keeper Kenny kept Forster at bay, blocking his shot with his legs, before Reading broke the deadlock in the 42nd minute.
The United back four dithered as Kitson headed the ball down on the edge of the box and Harper finished with a fierce strike beyond Kenny's reach.
Reading almost snatched a second a minute before the interval, Kitson pouncing on Bobby Convey's corner and screwing the ball into the ground before it bounced up and hit the bar.
United skipper Chris Morgan went close and Gray almost caught out Hahnemann, who spilled his long-range shot before the loose ball was scooped clear.
United brought on three substitutes but ran out of ideas and had to accept a defeat.