The Paisley side also prevented their opponents from recording their first league win at their new stadium.
Craig Dargo served notice of his threat on five minutes when his deflected shot from the by-line had to be smothered on the goal-line by Jamie Langfield.
Then Aberdeen striker Lee Miller somehow missed a header from three yards out, clipping the post from Charlie Mulgrew's cross.
At the other end Saints should have broken the deadlock on 16 minutes when livewire Dargo released Andy Dorman, however Langfield came out to block.
Neither side could get a grip on what was an untidy match, with falling sleet making conditions difficult.
Dargo did try a 20-yard shot on the half hour, and at the other end Lee Mair had an 18-yard shot held easily by the keeper.
Five minutes before the break Dons striker Miller hobbled off to be replaced by eventual hero Tommy Wright.
The second half opened up in brighter fashion with Langfield diving full length to save Dargo's shot.
On 53 minutes Jimmy Calderwood's side missed their second guilt-edged chance of the game with this time Darren Mackie the culprit, sliding in to send a point-blank shot over the bar after a dangerous cross from Chris Maguire.
On the hour, Mackie was again clean through from a Diamond lob that caught out the Saints' defence, however Chris Smith came out to bravely dive at his feet.
With 13 minutes to go, the stadium erupted when Dargo side-flicked home from five yards from a Billy Mehmet cross that caught out Diamond.
With two minutes left Saints made a lightening breakaway with Dorman sending David Barron clear, and he hit his shot powerfully, but Langfield was a match, parrying to safety.
Then three minutes into stoppage time, and with almost the last touch of the ball, Sone Aluko hit a cross from the left and Wright fired his header beyond the keeper for a last-gasp equaliser.