By Alasdair LamontSenior football reporter, BBC Scotland at St Mirren Park
St Mirren ended their own four-game winless league run while at the same time inflicting United's first defeat in seven games.
Danny Lennon's side went two goals up through Steven Thompson and John McGinn, but Nadir Ciftci reduced the deficit before the break.
United looked likely to equalise until Keith Watson was shown a red card for a bad foul on Sean Kelly.
Jason Naismith scored St Mirren's third and Thompson added another late on.
It was a highly entertaining and evenly-contested game for the most part, with St Mirren snapping into tackles to avoid United's talented young players settling into the game.
But St Mirren are not lacking talented players of their own and though Naismith just missed the target with a low drive, it wasn't long before they did have the lead.
Conor Newton knocked a cross from the left down to Thompson, who killed the ball before spinning to fire it low into Radoslaw Cierzniak's bottom left-hand corner.
And though United pressed for an equaliser, with Stuart Armstrong failing to convert from close range, it was St Mirren who scored again when McGinn pounced on Gavin Gunning's slip to tuck the ball past the goalkeeper.
Referee Bobby Madden gives United's Keith Watson his marching orders
The two-goal advantage did not hold for long though, as Ciftci latched onto a beautifully judged pass by Ryan Gauld to score, despite the combined efforts of Marian Kello, Darren McGregor and Marc McAusland.
St Mirren's captain Jim Goodwin was taken off just before half time, much to his displeasure, though as he'd been booked it looked to be a precautionary move by Danny Lennon.
It looked like it might backfire as United pressed for an equaliser, Kello brilliantly denying John Rankin, who fired in from distance, and Gary Mackay-Steven who tried to lob the keeper.
Then came Watson's dismissal and Naismith's goal within minutes of each other, the fullback nipping in at the back post to slide the ball past Cierzniak.
And with the heat taken out of the game, Thompson added a late fourth with a late drive past the keeper to seal a remarkable victory.
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