A closely fought tie came to life in the final 18 minutes as the Perth side struck twice in two minutes through Chris Kane, who flicked home a cross from substitute Glenn Middleton, who then doubled the lead with a stunning 25-yard free-kick.
An 85th minute header from Conor McCarthy pulled one back for the Paisley side and set up a tense finish but Callum Davidson’s men held on to reach their second final of the season, having lifted the Betfred Cup in February.
St Johnstone will now meet Hibs in the final on Saturday, May 22 after the Edinburgh side overcame Dundee United in Saturday’s other semi-final.
Here is how the two sets of Saints players rated ...
5. Ilkay Durmus - 7
An up and down game. Had periods where he was heavily involved then dropped out for large spells. Photo: Craig Foy - SNS Group
6. Ethan Erhahon - 8
Composed and accurate in possession and put in a strong shift off the ball as well. Photo: Craig Foy - SNS Group
7. Jake Doyle-Hayes - 8
St Mirren's best player. Terrific at breaking up play and good use of the ball as well. Photo: Craig Williamson - SNS Group
8. Jamie McGrath - 5
St Mirren needed a big game from their creative spark - other than the cross for the goal, they didn't get it. Photo: Craig Williamson - SNS Group