The midfielder fired home from the penalty spot on the stroke of half-time to snatch a share of the spoils on Friday, as both sides missed a chance to ease their fears of a relegation play-off in the Scottish Premiership.
Kris Doolan nodded home Kallum Higginbotham's 26th-minute cross to put Partick ahead before Aaron Taylor-Sinclair went close to doubling the advantage when he fired wide soon afterwards.
But when Higginbotham fouled Jason Naismith in the area just before the break, St Mirren were presented with a chance to level from the spot.
McLean kept his nerve, beating goalkeeper Paul Gallacher from 12 yards, for his seventh goal of the season.
The 22-year-old almost won it for the visitors late on, but his effort from outside the area cannoned off the right-hand post as St Mirren stayed a point ahead of Partick, who are now three points clear of the play-off spot.
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