It was a win which took the Terrors into third spot and maintained their impressive record in the SPL.
While the Tangerines did not play at their best, there was no need as the visitors created very little with United keeper Grzegorz Szamotulski not having a save to make throughout the game.
In the end, the winning margin should have been far greater as United dominated from the outset.
With only 22 seconds on the clock Willo Flood ploughed his way past three defenders before firing a curling shot which Chris Smith did well to tip over the bar.
As the Buddies tried to hold their shape, a piece of brilliance by Barry Robson in the 25th minute set up young Jordan Robertson with a six-yard header which the on-loan striker flicked wide.
However, the Terrors were rewarded for their pressure in the 38th minute when Noel Hunt was gifted a goal.
Cutting inside from a Sean Dillon pass, Robertson was gliding across the edge of the box looking for space to shoot when Franco Miranda slid in a tackle passing the ball back and straight into the path of the Irish striker who then had the easiest of tasks of rounding the keeper and stroking the ball into the net.
St Mirren's chances of a recovery were dented in the 67th minute when Miranda was sent packing for his second caution.
After being booked in the 28th minute for scything down Robson, the Argentinian committed the same sin again on the same player.
And the Paisley outfit were punished further a minute later when substitute Steven Robb collected a clearance from Robson's corner to cross low for Robertson to dink the ball over the keeper.
And it should have been 3-0 two minutes later when Hunt was sent racing clear by Flood, but as he rounded the keeper, he gave himself too tight an angle.
United were in full flow and in the 73rd minute Prince Bauben sent Robb clear but as he turned away, his celebrations were cut short when the ball struck the base of the post.
And the unlucky substitute could not believe his misfortune again eight minutes later when his wonderful volley from Hunt's cross rattled off the bar.