The Buddies started the brighter but were sucker-punched when Steven MacLean volleyed home at the back post from David Wotherspoon's corner seven minutes before half-time.
The defeat is St Mirren's seventh in their last eight league games and leaves them two-points adrift of Partick Thistle in 10th.
"I am flat," admitted dejected boss Danny Lennon. "Especially because of what is at stake.
"It was a very disappointing performance. The players know what is at stake, our livelihoods in the Scottish Premiership.
"But we have to make sure this club remains in this division. There have been too many bricks laid in the foundations here for that to fizzle out."
- Sports & Recreation
- St Mirren
- David Wotherspoon
- Partick Thistle
- Danny Lennon