This was Partick Thistle’s third consecutive win and it guarantees them top-flight status with only three league fixtures left to play.
After dominating the first half, Kallum Higginbotham’s cross in the second was headed in by Ryan Stevenson.
Kris Doolan then netted his 10th of the season with a half-volley.
And Steven Lawless added a low, powerful third from 20 yards in injury time to seal a convincing victory.
The physical strength and quick passing of Partick Thistle was too much for St Mirren to get to grips with for the majority of the first half. There was some excellent interplay between Higginbotham, Stevenson and Stephen O’Donnell. Lyle Taylor in his second spell at Thistle shot wide early on before Lawless, so often the creator for the Jags, saw two of his efforts go past a post and over the bar.
Even Belgian defender Fred Frans was trying to get in on the goal-scoring act for a second week in a row with a hooked attempt over the shoulder.
St Mirren’s one serious attack in the opening 45 minutes resulted in a strong penalty claim. Thomas Reilly’s perfect, lofted pass had Jason Naismith marauding into the box and the right-back appeared to be bundled over.
Referee Craig Thomson looked to his official for a flag but the pair’s conclusion was ‘no penalty’.
After the break, Steven Thompson almost made the most of a mix-up between Conrad Balatoni and Jags goalkeeper Scott Fox, the latter’s clearance being charged down by Thompson and spinning wide.
But from there on in Partick took control again. Stevenson rose against a previously sturdy Saints defence to head in his seventh of the season before striker Doolan showed his predatory instincts to convert well on the half-volley.
And Lawless finished excellently with an arrowed effort past Ridgers after a half-clearance.