The Buddies were offered a stay of execution despite suffering a heavy defeat because Motherwell failed to win against Ross County.
However, they now lie eight points behind the Steelmen with just three games to play.
But this was a deserved win for the home side which dragged Thistle clear of relegation trouble, with all three goals coming in the second half.
Strikes from Ryan Stevenson and substitute Kris Doolan, along with an injury time goal from Steven Lawless, ensured that the Jags will play top flight football again next season in a commanding performance.
The hosts made the most of the early running at Firhill with Lyle Taylor flashing a low effort just wide of Mark Ridgers’ post.
Lawless was next to threaten for the home side with two decent chances. His first effort was deflected just wide for a corner before he forced Ridgers into a great fingertip save after latching on to a Stevenson cut back.
Frederic Frans nearly broke the deadlock for Thistle in spectacular fashion but his overhead kick inside the box looped just wide of the post.
St Mirren then had big claims for a penalty waved away by referee Craig Thomson after Jason Naismith was seemingly brought down in the Thistle penalty area.
Steven Thompson nearly scored a fortuitous opener after the break for the visitors when he tried to take advantage of a defensive mix up between Scott Fox and Conrad Balatoni, with the Buddies captain charging down the clearance which ended up going just wide of the Thistle post.
The home side did grab the opener after 68 minutes, however, when Stevenson got on the end of a Kallum Higginbotham cross to head past Ridgers.
And Thistle made sure of the win when substitute Doolan turned nicely in the box before firing home to double his side’s lead.
The comprehensive result was wrapped up in injury time when Ridgers did well to deny Doolan, but the ball fell to Lawless who fired home from the edge of the box.