The penalty came after Matt Hill tripped Gylfi Sigurdsson, earning himself a yellow card in the process.
It meant Reading extended their fine run to eight wins in their last nine games. It was Neil Warnock's first loss in four games since taking over at QPR.
Warnock's influence on Rangers was soon apparent with his side's tight marking and battling attitude making life awkward for Reading.
QPR were effective in closing down Reading's dangerous wingers Jobi McAnuff and Jimmy Kebe. But their combative style did not meet with the approval of referee Gavin Ward, who dismissed Damion Stewart a minute before the interval after showing him two yellow cards.
Stewart had already been booked for a foul on Shane Long when he baulked Reading keeper Adam Federici as he tried to make a clearance, knocking him to the ground and collecting his second card.
Mr Ward had also booked Rangers Alejandro Faurlin for a first-half foul and Rangers skipper Mikele Leigertwood and Matthew Connolly in the second half. Reading's Grzegorz Rasiak was also booked.
The pattern of the game was set right from the start with Reading winning a free-kick for a foul by Matthew Connolly on Kebe, but Sigurdsson's free-kick was comfortably saved by Carl Ikeme. Brian Howard then sent another free-kick whistling just wide of the Londoners' post.
As QPR survived Reading's pressure they began to have the better chances themselves. There was a real scare for Reading when Federici made a fine point-blank save from a Kaspars Gorkss header and the ball fell to Priskin who fired over the bar when he could have scored.
As Rangers continued to press Adel Taarabt was the next to test Federici, who then followed that with a smart save to keep out Jay Simpson.
But in the end the task of holding out Reading with only ten men proved just too much for QPR.