Inverness defended extremely well and gained their reward with a penalty by Ian Black in injury time.
David Weir headed high in the air when he could have let the ball go through to keeper Allan McGregor and then compounded his error by pulling down David Proctor.
Weir was sent off for his troubles before Black coolly slid the spot-kick past McGregor.
This result allows Celtic to move three points clear at the top of the table with the Inverness side moving out of the bottom slot.
It was an insipid display by the Ibrox side, who lacked any creativity on the night.
Thistle were first to any loose ball and successfully harried the home team throughout the 90 minutes.
Rangers started brightly and Kris Boyd should have done better when a Steve Davis pass put him clear but the striker lost control.
Ryan Esson then saved well at the feet of Boyd before the Rangers striker shot against the keeper.
A deflected cross by Sasa Papac was cleared off the line.
Kenny Miller did have the ball in the net on the half-hour mark but was clearly offside.
Just before half-time, Papac nearly surprised Esson with a snapshot from the byline which the keeper did well to hold.
The longer the game went on the more Rangers ran out of ideas and resorted to aimless balls into the box which the Thistle defence cleared easily.
Midway through the second half, Kirk Broadfoot was crowded out at the far post before Boyd's header was saved by Esson.
The keeper made a brave save at the feet of Madjid Bougherra before Weir conceded that penalty right at the death, leading to a defeat which may be a crushing blow to Rangers' chances of wresting the league title from Celtic.