The home side moved seven points clear of the Steelmen who occupy the playoff spot in the Scottish Premiership thanks to two goals from Lyle Taylor.
And, despite a decent start from Motherwell, it was a well-deserved win for Thistle who also leapfrogged Kilmarnock into eighth place.
It was the away side who nearly went ahead after just two minutes when Abdul Osman’s backpass was intercepted by Scott McDonald who squared for John Sutton but his effort was deflected just wide.
Motherwell enjoyed the better of the early exchanges at Firhill as a swirling wind made it difficult for both sides. Lionel Ainsworth looked to carve out another Well chance when he crossed for Lee Erwin but his header was straight at Thistle goalkeeper Scott Fox.
The home side came into the game as the half wore on, however, as Osman’s shot was dealt with by Well goalkeeper George Long after taking a deflection on the half hour mark.
A few minutes later Taylor outpaced Stephen McManus before delivering a good ball into the box which was just out of reach for Thistle team mate Ryan Stevenson at the back post.
Taylor then carved out a chance for himself but could only fire over the bar before Kallum Higginbotham drilled a dangerous ball across the face of the Motherwell goal but there were no Thistle players there to take advantage.
Taylor had looked lively as the first half came to a close and he was the man who gave the home side the lead nearly 10 minutes into the second period. Higginbotham’s free kick was knocked on by Conrad Balatoni to Taylor who headed in from a yard out.
Thistle could have made it 2-0 straight after going in front but Stuart Bannigan could not pick out a man with his cut back.
The hosts were well on top and Taylor and Balatoni both had chances to extend their side’s lead but neither could beat Long.
Thistle’s dominance paid off shortly after, however, as Taylor got on the score sheet again, poking the ball home from close range after a Callum Booth delivery.
The home side went looking for more but Stephen O’Donnell saw his effort saved by Long before substitute Kris Doolan fired over the bar.
Motherwell endured further misery as the clock ticked down when substitute Stuart Carswell was shown his marching orders after picking up two yellow cards after just 13 minutes on the pitch.