Johnny Russell's second-half penalty gave Dundee United their first Scottish Premier League win since August as St Mirren suffered a third straight loss.
Lee Mair came closest to a first-half opener, the Buddies defender glancing a header wide.
United's Gavin Gunning nodded against the crossbar after the break.
And the visitors took the lead after Steven Thompson had been penalised for handball, Russell powering the ball home from 12 yards.
St Mirren replace United as the league's second-bottom side, with 10th-place United also on 12 points but boasting a better goal difference.
The first real chance came for the Tangerines with goalkeeper Craig Samson spilling Mark Millar's 19-yard shot before recovering the ball.
Mair glanced a header just wide of the United goal after meeting Dougie Imrie's corner and St Mirren captain Jim Goodwin was also off-target with a controlled 35-yard strike.
United had been restricted to long-range efforts and manager Peter Houston cut a frustrated figure in the visiting technical area.
Houston did have a potential ace up his sleeve in the form of new signing Rudi Skacel, scorer of 10 goals in six games against St Mirren for Hearts over the past two seasons, but the Czech midfielder's lack of match practice meant he would not appear until after the break.
St Mirren dominated possession but struggled to break the Tannadice side down and there was a sense a relief when the whistle sounded for half-time, with both sets of fans hoping for a better second half.
And Gunning almost broke the deadlock when he rose to head Willo Flood's free-kick onto the bar on 50 minutes.
Skacel and Richie Ryan came on in place of Millar and Michael Gardyne but United nearly found themselves behind when Lewis Guy turned the ball towards goal only for Radoslaw Cierzniak to smother the ball on the line.
Referee Euan Norris pointed to the spot after Thompson's outstretched limb made contact with Flood's corner and Samson was unable to prevent Russell's low strike finding the net.
St Mirren tried to respond and Goodwin's volley dipped into the arms of Cierzniak.
United held firm, albeit with some frantic defending as St Mirren shelled balls into the box.
Skacel almost got in for a scoring chance on the break late on but one goal proved to be enough for Houston's men.