Kris Doolan finally grabbed his 100th goal for the hosts but it was his 101st which consigned County to defeat, marking the end of their top-six ambitions and the start of their relegation worries.
The Highlanders have now taken only five points during 2017 – fewer than any other top-tier side – and they must now find a way to reverse – or at least arrest – their negative momentum when Dundee come to Dingwall on Tuesday.
Even taking the lead at Firhill did County little good as Doolan’s double left them only one point clear of second-bottom Hamilton.
“I’m obviously disappointed,” said manager Jim McIntyre. “I thought we were the better side first half; we had a really good chance to score just before half-time and, with the way things have been falling for us lately, it’s cracked off the bar.
“We came out second half and I was quite pleased with how it was going. We get the goal and then we seem to sit back.
“My main gripe is the free-kick for the first goal. Martin Woods clearly gets a touch on the ball but that doesn’t take away how we defended both goals.”
The opening exchanges were tentative, to say the least. Steven Lawless’s header from Ryan Edwards’ cross was comfortably gathered by former Thistle goalkeeper Scott Fox, while Tomas Cerny got down low to his left to hold an angled drive by Craig Curran.
County also might have had a penalty in the 37th minute when first Liam Lindsay, with a shove, and then Ryan Edwards, courtesy of a rash lunge, impeded Alex Schalk, but referee Willie Collum ignored their claims. “It was 100 per cent a penalty,” claimed McIntyre.
Then, on the stroke of half-time, Marcus Fraser finished off a move he had started by heading a cross from Boyce against the crossbar.
Largely, though, the opening 45 minutes were formless and gormless, to the point of being unwatchable.
Fortunately, the pace picked up after the break and Chris Erskine, who had earlier fluffed his lines after being sent clear by Lawless, forced a fine one-handed save from Fox with a low shot from the edge of the penalty area.
However, it was the visitors who broke the deadlock in the 58th minute. A shot by Liam Boyce was blocked but fell kindly for Curran, whose rising left-foot strike evaded the outstretched right hand of Cerny.
Thistle manager Alan Archibald sent on Ade Azeez for the under-performing Erskine and, within a minute, the home side had drawn level when Doolan latched on to a back-heeled pass by Lindsay, turned his marker and beat Fox with an emphatic strike from eight yards.
The keeper then did well to palm away a header by Lindsay as Thistle went for the winner.
It duly arrived 11 minutes from time and that man Doolan was both architect and assassin. He chipped the ball to Adam Barton and, when the midfielder nodded the ball back into his path, he smashed it, left-footed, behind Fox without breaking his stride.
“It’s about making memories,” said Doolan. “As a team, we want to establish ourselves in the history books at the club. All the boys want their names to be remembered and I think we’re heading in the right direction.
“I have scored some memorable goals over the years, but these are the big ones. We still have a few games to go to make the top six, of course, but this has gone a long way to helping us.
“The bigger picture is the club. If I’m scoring goals, we’re going to make the top six and that’s the be-all and end-all.”