Steven Thompson, John McGinn and Jason Naismith were all on target for the home team while United, who had Keith Watson dismissed, replied with a Nadir Ciftci goal.
Thompson opened the scoring on 12 minutes as he controlled a knock down from Conor Newton before turning to dispatch a low drive into the corner.
McGinn (35) doubled the advantage for St Mirren when he pounced on Gavin Gunning's slip to tuck the ball past keeper Radoslaw Cierzniak.
But United responded a minute later as Ciftci latched onto a weighted pass from Ryan Gauld to score, despite the combined efforts of Marian Kello, Darren McGregor and Marc McAusland.
United pressed for an equaliser and Marian Kello brilliantly denied John Rankin, who fired in from distance, before Gary Mackay-Steven was also thwarted by the Buddies keeper who saved his attempted lob.
Watson was dismissal with 20 minutes remaining for a reckless challenge on Sean Kelly and 60 seconds later Naismith nipped in at the back post to slide the ball past Cierzniak.
Thompson completed his brace and the scoring with a late drive past the keeper to seal end St Mirren's four-game winless run in the league.