It was an industrious performance from the visitors, who played with determination and vigour, giving them a deserved victory.
The tone of the game was set in the eighth minute when Mark Corcoran powered down the left flank, crossing for predator Craig Dargo who threw himself towards the ball but narrowly missed from six yards.
Seeing his team out-fought and out-played, Craig Levein rushed down from the stand to encourage his players to push forward, however it was the Paisley outfit who picked up every loose ball.
On 16 minutes Billy Mehmet came close with a low strike and then a couple of minutes later a blunder by keeper Lukasz Zaluska gave Dargo an open goal, but the normally reliable striker astonishingly hit the ball wide when it seemed easier to score.
It took the Tangerines 25 minutes to construct their first chance of the game and almost produce the opener.
A neat triangular move ended with Noel Hunt hitting a tremendous 18-yard shot which man-of-the-match Mark Howard did brilliantly to stop.
The Saints seemed to have joy down the flanks with Corcoran winning all the balls from Willo Flood. Just after the half hour he beat the stand-in right-back and was unlucky to see his shot well blocked by the keeper.
Approaching the break, against the run of play, the Terrors had two chances to take the lead. The first being a Hunt 20-yard free-kick which flew just past the post and then a thunderous Mark Kerr volley which was net-bound until being deflected away by the boot of Will Haining.
The second period was a mirror of the first with the Saints dominant and it was no surprise that they took the lead in the 48th minute.
Some slack defending from the United defence missed the opportunity to clear and the ball fell to Andy Dorman who clipped it over the advancing Zaluska.
From there on the ball was virtually always in St Mirren's half as United carved out chance after chance. In particular three glorious opportunities fell to Hunt, but symptomatic of United's play he contrived to waste every one of them.
Most notably five minutes from the end, Mark De Vries headed down and Hunt blasted the ball over with the goal gaping.
This, together with some fantastic saves from Howard, saw the evening end with the Tangerines being booed off and St Mirren celebrating looking forward to a quarter-final against St Johnstone.