Aidan Fitzpatrick put Thistle three ahead on aggregate two minutes before half-time in the second leg in Dingwall and they were in control until the 71st minute when Yan Dhanda’s penalty and Simon Murray’s close-range finish turned the game on its head inside 80 seconds.
County completed its comeback just after the 90-minute mark when George Harmon netted to send the game into extra time.
There were chances and VAR controversy throughout and the penalty shoot-out was just as dramatic as the game before Josh Sims finally settled matters three hours after kick-off to seal a 5-4 shoot-out victory.
Malky Mackay’s side extended its four-year stay in the top flight while Thistle, which went down through the play-offs in 2018 and have been to League One and back to the Championship since, will eventually find encouragement from its form under Kris Doolan once its shock subsides.
It was only a second defeat in 19 games under its manager, who suffered the loss of his father during a play-off campaign which saw Thistle win five matches and score 19 goals.
County made four changes, two of them enforced through suspension, as Ben Purrington came in for his first start since suffering a serious ankle injury in October. Doolan named the same team for a fifth game in a row.
County’s only real first-half pressure came courtesy of Thistle goalkeeper David Mitchell failing to deal with two high balls, Purrington seeing a header cleared off the line after the second blunder.
The home side survived a lengthy VAR review after Greg Aitken called referee Nick Walsh to his monitor after Ross Docherty went down under Nohan Kenneh’s challenge as he ran into the box. Walsh had a long look while County boss Mackay told him “it’s a dive”, and the referee decided against awarding the penalty despite definite contact.
The visitor soon took the lead after Tiffoney reacted quickest to a loose ball and drove it across the box. Fitzpatrick took a touch as David Cancola dived in, and the Thistle wide man reversed a finish past Ross Laidlaw.
The noisy visiting fans chanted “Thistle are back” and Alex Samuel missed an excellent chance to give the home support hope just after the break following a slip in the Jags defence. Mitchell made a good stop and a grounded Aaron Muirhead completed the clearance.
Thistle looked comfortable and Connor Randall cleared off the line from Steven Lawless as they looked to put the game out of sight.
But the complexion of the tie was transformed with 19 minutes of the 90 remaining when Aitken alerted Walsh to the ball hitting Graham’s arm in the Thistle box and Dhanda converted the resulting spot-kick.
Thistle then conceded a throw-in from the restart and Samuel capitalised on some hesitation from Darren Brownlie and drove in a cross which substitute Murray converted.
Jordan White and Brian Graham missed chances at either end before more VAR drama when County captain Keith Watson was shown a straight red card for hacking down Tiffoney from behind as the forward was about to thread a team-mate through. Walsh downgraded to a yellow after being called to the monitor.
The fourth official signalled for nine minutes of stoppage time just as Dhanda crossed for substitute Harmon to volley in off the post to make it 3-3 on aggregate. A VAR check for offside prolonged the tension.
There were further opportunities at either end and Murray missed a glorious chance with the last kick of normal time when he volleyed over from seven yards.
Samuel headed against the bar but Thistle had the better of extra time and Stuart Bannigan blazed over the best chance after being set up by Graham inside the six-yard box.
The shoot-out was just as tense.
Kevin Holt and Jack Baldwin saw penalties saved for either team before Harmon’s pressure kick took it into sudden death. Bannigan hit the post before Watson blazed over, and Laidlaw saved from Docherty before Sims netted to spark a pitch invasion from the home fans.