Saulius Mikoliunas curled a superb left-foot shot past Chris Smith and just inside the post to equalise.
It set up a frantic finish to the match, but Stephen Frail's side somehow survived the late Buddies onslaught.
The opening exchanges produced few opportunities, until late in the first half when St Mirren keeper Smith, produced a brilliant double save, firstly pushing Andrew Driver's effort out for a corner, and from the resultant kick going down on his line to hold a Hristos Karipidis header.
Saints by this stage were reeling after the loss of defender John Potter, who took a severe head knock when trying to head clear, from a wayward boot by Hearts player Mikoliunas.
At the start of the second half Hearts almost grabbed the lead, but again it was Smith to the home side's rescue with a great save to push over Calum Elliot's rising shot.
On 53 minutes Garcia Ruben Palazuelos took advantage of a slip from David van Zanten and released Elliot, but this time he shot wide from an angled drive.
On 58 minutes St Mirren grabbed the lead, with a goal totally unglamorous.-Ian Maxwell's free kick was missed in the box by Hearts defender Hristos Karipidis.
Craig Dargo took full advantage and touched the ball into the path of one-time Hearts player Jim Hamilton and he smashed his first St Mirren goal high into the top corner.
A terrific forward run by Hearts' Lee Wallace set up Elliot again, but once more he was off-target.
St Mirren almost wrapped up the points on 70 minutes when brilliant play between Andy Dorman and Dargo saw the former go through and beat Steve Banks with a great shot, only to see the ball thump off the post before Hearts cleared at the second attempt.
With nine minutes remaining Dargo was found in space by Hugh Murray, but with Hamilton waiting to pounce, Banks made a good save.
Hearts then grabbed the all-important equaliser with four minutes remaining, from Mikoliunas's spinning shot, but this seemed to spur on St Mirren and in particular Dorman.
The January signing had three great chances to score in the last two minutes, but failed every time, twice missing the target and once being denied by Banks.