Agyemang turned the ball in from close range with six minutes remaining at Ashton Gate after Jon Stead's superb 16th-minute strike looked like it would gave bottom-of-the-table City victory.
Agyemang's goal saw Rangers set a new club record of 13 league games unbeaten from the start of the season, and extend their lead at the top of the table to three points over Cardiff, who travel to Leeds on Monday night.
Meanwhile, City remain rooted to the foot of the division.
Robins boss Keith Millen was keen to name an unchanged side after recording a first home win of the season against Reading on Tuesday night, but Jamal Campbell-Ryce failed a fitness test on a hamstring problem.
On-loan Tottenham midfielder Danny Rose was recalled in his absence, while QPR were unchanged.
Jamie Mackie had the first chance after 10 minutes but the Championship's joint-leading scorer failed to add to his eight goals this season and dragged his shot across goal.
Heidar Helguson was inches away from connecting with Hogan Ephraim's low cross soon afterwards.
It was somewhat against the run of play when Stead struck. It stemmed from Adel Taarabt's poor delivery from a free-kick that was cleared on the edge of the City box.
Rose led the breakaway before slipping the ball to Stead, who beat Paddy Kenny for only the fourth time this season with a 22-yard shot that crashed in off the underside of the crossbar.
The goal lifted the Robins and Rose forced Kenny to save with his feet from a tight angle after 20 minutes.
Rose continued to make a nuisance of himself and only a deft flick from the outstretched foot of Clint Hill prevented the Tottenham man from setting up a second goal for Stead.
QPR came alive again just before the break and Kaspars Gorkss headed just over before Kyle Walker whistled a shot past the far post after cutting inside from the flank.
Neil Warnock's side maintained their momentum at the start of the second half and thought they had equalised straight from the restart when Mackie headed home the rebound after David James saved from Helguson.
However, Helguson was unable to get out of the way of Mackie's effort and the ball went in via the Icelander's thigh while he was stood in an offside position.
Immediately, at the other end, Haynes could have extended City's advantage but scuffed his shot wide from 10 yards out.
Kenny denied Marvin Elliott and Stead in quick succession midway through the second half to keep QPR in the game.
Some concerted Rangers pressure almost culminated in an equaliser but Matthew Connolly's header was cleared off the line by Jamie McAllister after 72 minutes.
Haynes had another chance to put the game beyond the league leaders 10 minutes later but dragged his shot wide once more.
It proved a costly miss as Connolly flicked on Tommy Smith's low corner and Agyemang, a 78th-minute replacement for Taarabt, turned the ball in from a yard out.