The visiting fans were in jubilant mood with their side comfortably ahead and St Mirren trailing 2-1 at home to Motherwell with just two minutes remaining.
But two goals in the final minutes at Paisley - from Kenny McLean and Steven Thompson - secured all three points for Danny Lennon's men and consigned the Jambos to Championship football next season.
The outcome was cruel on Gary Locke's side who recovered from a shaky start to give their vocal band of travelling fans hope of taking their fight into the final post-split fixtures of the campaign.
The opening exchanges gave little indication as to the outcome of the game after Kris Doolan fired Thistle in front with just five minutes on the clock.
But after surviving a shaky period, where they could have slipped further behind on more than one occasion, a goal on the stroke of half-time, and a second half blitz, had their fans believing the battle could be taken into the remaining five games of the campaign.
Doolan took advantage of hesitancy in the Hearts' rearguard to open the scoring but Dale Carrick ensured the sides would go in level when heading home on the stroke of half-time following fine work by Billy King.
Five minutes into the second period King rifled an unstoppable shot beyond keeper Paul Gallacher and the wild celebrations in the visiting end had barely died down when Ryan Stevenson headed a third shortly after the hour mark.
Thistle looked visibly stunned and any fight left in Alan Archibald's side frittered away when Stevenson shook off his marker and gave Gallacher no chance with an angled drive from 15 yards out.
Jordan McMillan grabbed a mere consolation for the Jags two minutes from time before news of the thrilling finish at St Mirren Park ensured fans from both sides trooped out of Maryhill in the bleakest of moods.
Hearts, who were docked 15 points at the start of the season after entering administration, have just 13 points to their name, 17 behind second-bottom Thistle who have now dropped into the relegation play-off spot.