It was shattering for St Mirren after they had clawed their way back from a two-goal deficit and looked to be heading for a replay as United's much maligned defence had appeared to have blown it again.
The game had begun in a dull fashion when the visitors clocked up the opener in the 12th minute only for the flag of the assistant to chalk off John Sutton's excellent header.
An immediate route one response from the Tangerines saw Collin Samuels' 20-yard strike flash past the post when it appeared to be heading in.
But with the Saints pushing forward more and more, Alan Archibald was fortunate not to be penalised in the box for blatant holding of Sutton.
The Terrors appeared to strike their opener in the 21st minute when Barry Robson's corner was forced in by a combination of Samuel and Robertson but, despite the claims for a goal, Saints keeper Tony Bullock appeared out of the melee with the ball and the flag stayed down.
United had moved up a gear. Barry Robson tried an adventurous 20-yard lob in the 28th minute which whistled just over the bar.
The captain was justly rewarded in the 31st minute when he curled in a 25-yard beauty which deceived Bullock with its vicious swerve.
The Terrors appeared to head their way into the next round with a Garry Kenneth bullet header 50 seconds into the second period.
Saints made an attacking change in the 56th minute with Stewart Kean coming on for the tiring Andy Millen and the substitute should have equalised in the following minute but headed over an empty goal.
But the visitors were given an unlikely lifeline in the 76th minute when home debutant Jon Daly tripped Kirk Broadfoot. It was a soft penalty but was converted with venom by Richard Brittain.
Astonishingly, the Saints equalised in the 81st minute when the ball broke fortuitously to Broadfoot who passed to Sutton who made no mistake of coolly sliding the ball past Stillie from 16 yards.
The fans were heading for the exits when Robertson struck the sword into the hearts of the Saints.
John Potter tried to shepherd the ball out over the byline only for Samuel to stick a leg and pass the ball to Conway whose cross was sliced in from 12 yards.