Jordan Slew and Yoann Arquin had put County 2-0 up before Lyle Taylor and George Moncour brought Thistle level but De Leuw’s deflected effort won the day and ensured County finished in seventh.
There was some good early play between Chris Erskine and Kallum Higginbotham before the former fired the ball across the face of goal but Taylor could not connect.
Slew surged through the Thistle defence and shot narrowly wide but moments later the County striker – on loan from Blackburn Rovers – beat the offside trap and slotted his first goal for County past Scott Fox (22).
The Thistle goalkeeper was under pressure again from Slew as he tried to punch clear and fell awkwardly. After a a lengthy delay he was replaced by Paul Gallacher.
Conrad Balatoni went close for Thistle before Arquin (48) rose completely unmarked at the far post to plant his header past the substitute keeper.
A great cross from Chris Elliott allowed Taylor (51) to rise and head home for Thistle and Higginbotham then went close as the home side gathered momentum.
Mark Brown pulled off an excellent save from Steven Lawless’ shot after good work by Higginbotham but the leveller did arrive when Taylor cleverly set up George Moncour (73) to slot home from 16 yards.
De Leeuw then worked some space in the Thistle box (77) and his left-foot shot deflected off Balatoni to give Gallacher no chance and seal the win.