St Mirren led on 37 minutes when Darren McGregor's header from a Jure Travner corner was knocked clear by United's Morgaro Gomis, but the officials ruled the ball had already crossed the line.
David Goodwillie should have claimed an earlier equaliser for United, who dominated much of the game, but he sent a close-range shot against the crossbar when it seemed easier to score.
It typified a night of wasteful finishing by United, who were also finding home goalkeeper Paul Gallacher in fine form, but Robertson's last-gasp header at least got them a point.
Dundee United were without suspended duo Jon Daly and Scott Robertson after both players were red-carded during last Saturday's 1-1 draw at Kilmarnock.
Stuart Armstrong, a substitute in that match, earned a start while Garry Kenneth was recalled for United against an unchanged St Mirren team.
Good defensive work in midfield by Gomis helped set up Goodwillie for an early shooting chance, but Gallacher was alert to the danger.
Craig Conway's pace down the left flank troubled St Mirren from the outset and from his low cross, Johnny Russell slid an effort across the face of goal.
The home team were offering little in response and a wayward shot wide from distance by Paul McGowan on 12 minutes summed up their efforts.
McGowan was more effective five minutes later when he set up Marc McAusland for a powerful driven effort that fizzed narrowly wide of the mark.
Goodwillie was booked for a needless handball before United attacked again when Barry Douglas sent in a cross that Russell connected with to force another Gallacher save.
McGowan, on loan from Celtic, was looking lively for St Mirren and after a neat Michael Higdon lay-off he fired a low drive that Dusan Pernis was equal to in the United goal.
United still looked the more likely to open the scoring, first Kenneth heading over from a Conway free kick then Gomis sending a swerving shot wide from distance.
It was St Mirren, however, who broke the deadlock on 37 minutes, Travner's corner headed in by McGregor and over the line before Gomis could hook the ball clear.
United came close to levelling in the 41st minute when another Conway cross was met by Goodwillie whose looping header clipped the top of the crossbar.
Travner was booked for upending Goodwillie in the 49th minute and the resulting Douglas free kick broke for Prince Buaben to shoot over from the edge of the area.
Gallacher was into action again on 52 minutes when, from a Douglas cross, Goodwillie's glancing header was pushed wide by the busy St Mirren goalkeeper.
United claimed for a penalty when Kenneth tumbled to the ground under challenge from McGregor, but there seemed minimal contact and nothing was given.
Kenneth got himself a warning from referee Craig Thomson after complaining again on the hour mark after again wanting a spot-kick this time claiming he was fouled by Lee Mair.
St Mirren rightly sensed they needed a second goal to secure the win, but Travner shot into the side-netting, while a McAusland volley was well-blocked by Kenneth.
Paddy Cregg was caught in possession for St Mirren on 73 minutes allowing Russell a run down the left and from his low cross Goodwillie conspired to hit the bar from point blank range.
It was a glaring miss, but it was not to be Goodwillie's last, he should have done better with his next attempt, a headed effort from a cross by substitute Robertson.
Another United replacement, Danny Cadamarteri, was next to threaten, but his angled drive bounced wide as St Mirren defended in numbers to preserve their slender advantage.
They were undone in the 87th minute when Robertson's looping header from Goodwillie's cross deceived Gallacher for what was a deserved equaliser on the balance of play.