Until then it looked as if Eggert Jonsson's 78th-minute strike would be enough to settle an untidy encounter.
But Saints - playing their second match in 48 hours - had other ideas and took their reward at the death.
Christos Karipidis was ruled out due to a virus and, with Marius Zaliukas still suspended, it meant Robbie Neilson was moved to central defence.
But it left Hearts short of height in central defence and Billy Mehmet looked eager to take advantage in the early stages.
It was Hearts who had the first effort on goal when David Obua was allowed to gallop from his own half to the edge of the Saints' area before sending a low, left-foot shot a couple of yards wide.
St Mirren were guilty of backing off again when Lee Wallace sent a tame shot wide when he had time and space to do better.
The makeshift Hearts defence was at sixes and sevens after quarter-of-an-hour when Dorman was left unmarked when Mehmet steered a header into the heart of the box but the home side managed to scramble it clear.
Saints came close again when Jim Hamilton got his head to a Dorman free-kick at the near post but the ball drifted wide.
It sparked something from Hearts. Chris Smith parried a shot from Christian Nade and Jack Ross hooked it wide for a corner.
Jonsson stuck out a toe to get on the end of Andrew Driver's flag-kick but the ball struck the post before Nade made an instinctive lunge at the rebound only to send it wide.
St Mirren then failed to deal with a deep free-kick from Neilson and when the ball broke to the edge of the box, the alert Bruno Aguiar volleyed wide.
Michael Stewart was fortunate not to be punished when he dallied on the edge of his own area but Hamilton sent his shot high and wide.
Hearts hit back and Obua tried his luck again from distance but Smith had it covered and saved comfortably.
Aguiar had the best effort of the first half with a superbly-struck free-kick which Smith turned over the top. Referee David Somers was the only man in the stadium who did not see the save and awarded a goal kick.
Chances were few and far between in the second half but Hearts snatched the lead after 78 minutes when Saints failed to deal with an Obua cross and Jonsson squeezed the ball home at the far post.
But Saints hit back in stoppage time and Dorman applied a close-range finish after Hamilton had headed the ball on to give them a point.