The 22-year-old left Bramall Lane as a youngster and returned for £1million a year ago.
He claimed two goals in the first 13 minutes and completed his treble early in the second half against the clueless Londoners.
Midfielder Greg Halford hurled in a long free-kick and striker Darius Henderson flick-headed the ball into the danger area and Sharp found space to stoop and place his accurate header out of Radek Cerny's reach after three minutes.
Two minutes later Henderson was helping out at the other end clearing after Kaspars Gorkss tried to squeeze a header in from a Daniel Parejo corner.
But the Sheffield club, beaten by an injury time goal from promotion favourites Birmingham last week, struck again.
Former Blades midfielder Mikele Leigertwood dithered and veteran Gary Speed stole the ball.
He angled an astute ball for Sharp who raced into space leaving Rangers trailing and rifled a cool shot past Cerny after 13 minutes.
The home side might have stretched their lead after 22 minutes when former Manchester City favourite Sun Jihai crossed and found Henderson, who headed down from close range but Cerny dived well to his left and held on.
United suffered a blow 11 minutes before half-time, when skipper Chris Morgan reluctantly bowed out with concussion under orders from the United bench and club doctor.
Three minutes before the break Sharp was within a whisker of a hat-trick after the smallest player on the pitch, Stephen Quinn, embarrassed the Rangers defence heading down for the striker whose angled shot was a fraction wide.
A minute later QPR's blushes were spared by a super save from Czech Republic international keeper Cerny.
Michael Tonge's right-foot shot from inside the area hit a crowd of players and bounced straight back to him.
His second attempt with the left foot was arrowing in, but Cerny made an impressive stop despite being unsighted.
And six minutes into the second half Sharp finally added his third goal when he stabbed the ball in from close range after Cerny flapped at a Tonge corner.