A double from Kris Doolan ensured Inverness suffered their first home defeat of the season in a controversial match in the Highlands.
Doolan put Partick ahead, firing low under Dean Brill after 20 minutes.
Aaron Doran thought he had levelled, but the goal was ruled out after officials ordered a corner be re-taken.
Gary Warren's header restored parity after the break, but hosts goalkeeper Brill fumbled a cross to give Doolan a simple tap-in to claim the win.
With Partick having encounters against Aberdeen and Celtic to come, manager Alan Archibald will have been delighted to grab a vitally important victory.
But where luck had perhaps evaded them in recent weeks, they certainly got the fortune in this game, with a contentious refereeing decision and Brill's gift to Doolan shortly before the end.
Second-placed Inverness will have felt aggrieved to have the lost the match with Celtic dropping points on Saturday.
However, on the evidence of the opening period, they might have thought their home record would be safe as they took the game to the visitors.
Doolan (right) netted both Partick Thistle goals
Doran came close with a free-kick before Ben Greenhalgh, starting his first game and eager to impress, had an effort cannon off the post.
However, it was Doolan, having received a James Craigen cutback, who showed the coolness and composure to fizz a shot past Brill to give the Jags the lead.
Partick were quite content to let Inverness have possession, but Archibald's men threatened on the counter on several occasions.
Craigen had time and space on the edge of the box and attempted to curl a shot towards goal, but he failed to find the target.
Stephen O'Donnell was next to try his luck as they flooded forward once more, but his effort was also wayward.
The game's big talking point arose just before half-time when Inverness thought they had found an equaliser.
A training-ground corner led to Doran dinking the ball across to Billy McKay, whose effort was cleared off the line before Doran rifled past a flurry of defenders and Scott Fox in goal.
However, referee Euan Norris disallowed the strike and brought play back, insisting that Ross Draper's initial touch at the corner flag had not restarted play, much to the fury of the Inverness players.
Their feeling of injustice spurred them on, though, and it was not long before they found an equaliser shortly after the break when Warren powered a header beyond Fox.
It seemed as though Inverness were starting to assert their authority on the game as Partick were struggling to deal with the aerial power they posed.
They looked vulnerable when Richie Foran rose above Isaac Osbourne to head into the net, but the official blew his whistle for an infringement.
Inverness spurned more chances through Draper and Foran as they looked to keep their 100% home record intact.
However, it was a howler from Brill that gave Doolan the simplest of chances to knock the ball in from a yard and give Partick the three points.
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