Russell sent a looping header into the far corner from Keith Watson's 84th-minute cross as Peter Houston's side moved level on points with Motherwell in seventh place following their fourth consecutive Scottish Premier League match without defeat.
Motherwell striker Henrik Ojamaa had twice hit the frame of the goal as the hosts dominated the middle part of the contest, but United came into it in the final quarter and snatched victory in a game originally postponed because of a power cut.
The 3,941 fans who turned out again witnessed some decent attacking play from both teams but Ojamaa found luck against him as he continued to seek his first goal of the season.
Jamie Murphy scored the first of his two goals against St Johnstone on Saturday inside the first 40 seconds and had a shot on target even earlier tonight, but Radoslaw Cierzniak got down to hold his long-range drive.
Murphy, who was back on the left wing after a central starting role in Perth, soon had a better chance when Michael Higdon laid Nicky Law's cross onto his feet eight yards out but Cierzniak produced a brilliant one-handed stop.
United, with Brian McLean in for the absent Barry Douglas, had their early chances too and almost caught Motherwell on the break but Stuart Armstrong was off target.
Jon Daly then met Keith Watson's cross but headed wide.
Murphy was continuing to cause problems on the left but Darren Randolph kept Motherwell level with an impressive double save, diving to his right to stop Gavin Gunning's powerful 25-yard free-kick before getting up to block Armstrong's follow-up.
Tom Hateley threatened from long range and Motherwell dominated the latter half of the first period with some measured possession play.
But United survived, although only just when Ojamaa ran at the visiting defence and unleashed a powerful 20-yard strike that crashed off the underside of the bar and bounced to safety.
Another Ojamaa shot suffered the same fate in the 57th minute. Keith Lasley won the ball from Willo Flood following a United throw and tried to play Ojamaa in on the right channel.
The pass forced Ojamaa wider than he would have liked but he still managed to beat Cierzniak with an angled drive only to see his effort bounce off the far post.
Motherwell soon had a decent penalty claim when Watson barged Murphy into the advertising hoardings behind the goal, but referee Alan Muir awarded a goal-kick.
Motherwell soon had two close escapes, the first when Armstrong's shot took a deflection and spun just wide.
United almost scored from Flood's corner when Russell helped the ball through a crowded goalmouth. The shot escaped Randolph's clutches but Lasley stopped it going in, although United fans behind the goal claimed that the ball had gone over the line.
Randolph then saved from Armstrong as United kept up the pressure with the hosts surviving some dangerous crosses.
The hosts had a good chance with 15 minutes left when Chris Humphrey drove from his own half into the box and laid a square ball into Higdon, who scuffed his shot and allowed Cierzniak to save.
Humphrey again found space in the box but his low cross was too far in front of his team-mates.
Neither side looked content with a point and it was United who made the breakthrough. Watson sent in a cross from just inside the Motherwell half over the head of Motherwell centre-back Adam Cummins and Russell looped a header from a tight angle over the head of Randolph and into the far corner.