After losing their two previous matches it was crucial for United to regain their impetus that has driven them into the top half of the table.
And after David Robertson volleyed them in front four minutes into the second half they looked on easy street. Noel Hunt added to United's advantage midway through the second half before Willo Flood put the final nail in the Saints' coffin with a stunning late drive.
Both teams began with a lone striker leading to a congested midfield.
Indeed, it took 15 minutes for either side to manage a shot at goal and Mark Corcoran was unfortunate to see his drive deflect wide off team-mate Billy Mehmet.
But the home side had a narrow escape after 22 minutes when Chris Smith dropped Morgaro Gomis' cross.
The ball fell to Darren Dods, but the keeper redeemed himself with a stunning save to his right and somehow Robertson fired the rebound high and wide when he looked certain to score.
Corcoran was St Mirren's most menacing player and he cut-in off the right wing twice, but both times failed to get the shots on target.
Another flap by Smith allowed Dods to head on Barry Robson's cross but the defender's header was a foot too high.
After a dismal first half it was difficult to see where goals would come from but United opened the scoring after 49 minutes to spark the match to life.
And Robson was the creator with a deep cross that Robertson stretched to volley home.
And with United getting plenty of midfield support to Hunt it always looked like they would increase their advantage.
And the second goal came after 77 minutes when Flood's free-kick was met by Dods. Smith parried the header, but Hunt was on hand to ram home the rebound from close range.
St Mirren's cause was further hampered when Hugh Murray picked up two quick yellow cards and left his side with ten men for the last ten minutes.
United took advantage of their superior numbers to round off the scoring on the stroke of full-time.
Prince Buaben galloped down the left and rolled the ball across the 18-yard line for the on-rushing Flood to crash home a low drive.