St Johnstone let slip a third league win in a row, after conceding a soft 90th minute equaliser.
They had taken the lead in the eighth minute through top scorer Stevie May.
His low strike from the edge of the box, which took a slight deflection off Lee Mair, brought his tally for the season to 23 (18 in the Premiership).
At that stage it looked as if the home side would run out comfortable winners, as they had started the match in total control.
But it didn’t pan out that way, and relegation-threatened Thistle deserved their point.
On-loan Dundee United man Chris Erskine had three good chances, but didn’t find the target with any of them.
The nearest Saints came to doubling their lead was when May headed the ball against the bar on 20 minutes after he followed up a Lee Croft shot that was blocked by Paul Gallacher.
Though Saints weren’t at their best, it looked like they would see the game out.
However in the last minute of regulation time Alan Mannus let a Kris Doolan shot, that he would normally have stopped, get past him.
