In a stormy and controversial match, Hearts laid siege on the Saints goal for much of the 90 minutes but found keeper Tony Bullock in totally inspired form.
To make things worse, Hearts also had two good penalty claims waved aside in the first half.
Saulius Mikoliunas wasted a glorious chance from Neil McCann's cross which found him unmarked in the box.
His low effort was brilliantly turned around the post by the briiliant Bullock, who then made a superb fingertip save from Steven Pressley's header to keep Saints alive in a one-sided first half.
Garry Brady and John Sutton then combined to create a chance for Stewart Kean which Craig Gordon parried to his left.
But in truth it was all Hearts throughout and Roman Bednar skipped around Bullock only for Andy Millen to get back and clear off the line as the pattern of the match continued to be set by the home side.
However, Saints grabbed the glory goal and sent Tynecastle silent just eight minutes from the final whistle.
Craig Molloy fired in a powerful shot from 15 yards which Gordon did well to parry but Kean reacted quickest to prod home and give St Mirren the most unlikely of victories.