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 ...
9. Richard Tait - 6
Both his crosses and his attempts to attack the ball inside the penalty box always seemed a yard off. Photo: Craig Foy - SNS Group
10. Kristian Dennis - 5
Looked lively in parts but another who dropped out of action for long spells. Photo: Craig Williamson - SNS Group
11. Lee Erwin - 5
Unlucky not to score but often didn't do enough to get St Mirren higher up the park. Conceded possession in build-up to opener too. Photo: Craig Foy - SNS Group
12. Collin Quaner - 3
Oh dear. Missed two sitters and had trouble trapping the ball. Photo: Craig Williamson - SNS Group