New United signing Rudi Skacel was handed the final 25 minutes but failed to add to his impressive strike record against Saints, having previously netted 10 goals in two seasons against the Paisley club while at Hearts.
The Czech winger started on the bench after signing a short-term deal with the Tannadice outfit on Friday, after training with former club Hearts, who are now under a transfer embargo until Christmas.
Saints made two changes from the side which lost 5-0 to Celtic last week, with Jon Robertson and Dougie Imrie drafted into the side in place of David Barron and Kenny McLean, who was ruled out with a knee injury.
The first real chance of the match fell to United when Mark Millar tried his luck with a shot from the edge of the area, which was spilled by Craig Samson, before the goalkeeper recovered to smother.
At the other end, an Imrie corner found the head of Lee Mair in the packed goal-mouth but his close-range header flashed wide of the target.
Saints continued to push for the opener and a clearance from Jon Daly - the striker was again deployed in defence - fell for Jim Goodwin, who rifled just over from 35 yards.
What followed for the remainder of the first half was little more than half chances as the two sides cancelled each other out and both sets of fans grew increasingly frustrated as half-time approached with no sign of an opening goal.
The start of the second half offered more promise, with United rattling the woodwork shortly after the restart.
A Willo Flood free-kick found Gavin Gunning and his header from eight yards crashed off the crossbar before he pounced on the rebound and lashed past the post.
The home fans screamed for a penalty on the hour mark when Lewis Guy went to ground under pressure from Gunning, but a spot-kick would have been harsh on United and referee Euan Norris signalled for play to continue.
Skacel made his debut for the visitors with 65 minutes on the clock when he was thrown into the action for Michael Gardyne, as Richie Ryan replaced Millar.
The breakthrough came four minutes later when Steven Thompson was judged to have handled a Gunning header in the box and the referee pointed to the spot.
Russell stepped up to send a low strike into the back of the net, with Samson getting a hand to the ball but unable to prevent the shot from crossing the line.
The Buddies tried to haul themselves back into the game and a volley from Goodwin from the edge of the area dipped over the crossbar.
The same player then squandered another chance to claim the equaliser when he hooked wide with a couple of minutes to go as United held on for the win.