QPR played the entire second half with 10 men on Tuesday night after having Damion Stewart sent off just before the interval, but they looked set to hang on for a point until Sigurdsson's 85th-minute spot-kick.
Warnock's side were the better side in the first half but Brian McDermott's side kept going after the break and eventually earned their eighth win in their last nine league games to continue their climb up the table.
Sigurdsson and Brian Howard had gone close with early free-kicks for the hosts but QPR bossed the rest of the opening period.
Warnock fielded a very attacking side, with captain Mikele Leigertwood often left in midfield on his own as his team-mates bombed forward.
Taarabt have one fierce effort well held by Adam Federici and Tamas Priskin should have put the visitors ahead when he blasted over the top when Federici could only parry a header from Kaspars Gorkss into the goalmouth.
The lively Taarabt then hammered a drive over the crossbar after showing a great touch before Jay Simpson added to the list of missed QPR chances.
Taarabt was again involved, and after drifting into space on the left the Tottenham loanee disguised a deft pass in behind the defence for Simpson to run onto but he could not lift the ball over Federici.
Rangers had been on top but the rest of the first half descended into a niggly battle after Stewart was booked for a high challenge on Shane Long which had Warnock fuming on the touchline.
Both sides were guilty of some cynical challenges as the crowd grew frustrated before Stewart was sent-off in the final minute of the half for senselessly challenging Federici as the goalkeeper took a quick drop-kick.
Leigertwood shifted to right-back as Rangers reshuffled for the second-half and Reading started brightly.
Jimmy Kebe hit a shot on the run straight at Ikeme before Long sent a curling effort from the edge of the area narrowly wide.
Ryan Bertrand's fizzed cross from the left evaded everyone when the merest of touches would have put the Royals ahead before Simpson missed another good chance for the visitors when he fired over from a tight angle.
Sigurdsson saw a powerful shot well cleared by Matt Hill from on the goal line as the home side continued to press - before Jobi McAnuff was denied by an even better clearance after his effort had beaten Ikeme.
But the Royals nicked it when Sigurdsson tricked his way into the area before being tripped and then blasting the resulting spot-kick into the top corner.