Following a goalless first half, the visitors took the lead after 68 minutes when substitute Andy O’Connell accepted a pass from Craig McGuffie to score his first goal for the Ayrshire club.
However, Sutton turned the match around by scoring twice in the final seven minutes.
First, the striker equalised with a volley from Stevie Mallan’s long ball. Two minutes later, he won the match for the Buddies when he poked the ball home.
It was manager Jack Ross’s first victory in five attempts since his appointment as St Mirren joined Dundee United and Queen of the South in the semi-finals