Substitute Colin Samuel's low strike four minutes from time earned United a deserved point.
'Well goalscorer Jim Hamilton (left) battles for the ball
Motherwell took the lead after they were awarded a penalty when Charlie Mulgrew hauled back Paul Quinn.
Jim Hamilton's spot-kick was brilliantly saved by Craig Samson but the striker was able to stab the ball home at the second attempt.
However, Samuel's late strike prevented United slipping to their seventh straight defeat of the season.
Motherwell's build up to their final match of the season was overshadowed by the speculation linking their manager Terry Butcher with a move to Australian A-League champions Sydney FC.
The Fir Park side could have taken an early lead when Jim Paterson's third-minute corner was punched clear by Samson to the feet of Alan McCormack at the edge of the box but he fluffed his shot completely.
Two minutes later United went close when Barry Robson crossed from the right and Jim McIntyre stabbed a left-foot effort narrowly wide.
The game had an end-of-season feel in the early stages, with a lack of urgency, despite some bright football.
'Well defender Steven Hammell did well to head a dangerous Mulgrew cross clear in front of goal on 21 minutes under pressure from McIntyre.
United passed up another opportunity shortly after when Robson was played in on the right side of the box, but he dragged a left-foot effort beyond Graeme Smith's far post.
Motherwell threatened to score on the half-hour mark when David Clarkson squared from the left for Martyn Corrigan, but his left-foot shot was blocked under pressure from Garry Kenneth and David McCracken.
Corrigan was subsequently stretchered off as a result of the clash, with Scotland squad keeper Smith also replaced by Colin Meldrum after picking up an earlier knock.
United came closest to opening the scoring a few minutes later, McIntyre crossed from the right and Lee Miller sent a header crashing off the crossbar.
On-loan Celtic striker Mulgrew had a chance to open for the visitors a minute after the break, when Robson's free-kick was cleared to the feet of the winger but he fired over.
Paterson was sent clear on the left by Hamilton, after fine work from David Clarkson. But the defender skewed his shot wide.
The home side took the lead in the 58th minute when Mulgrew hauled down Quinn in the box and referee Stuart Dougal awarded a penalty.
Hamilton's spot-kick was brilliantly pushed on to the post by Samson, before rolling back across the face of goal, but Kevin McBride raced in to knock the loose ball square for Hamilton to stab home.
United almost hit back in 62 minutes when Robson tried an ambitious curling effort from 30 yards which almost dipped into Meldrum's top corner.
Motherwell should have doubled their lead in 77 minutes after great work from Clarkson saw him slip Paterson clear on the left side of the box, but his shot was saved by the legs of Samson.
United pulled level after 86 minutes when Robson's flick-on put Samuel clear and he cut inside a defender before sending a right-footed shot low past Meldrum.
The final whistle was met with a pitch invasion by Well fans.
Motherwell: Graeme Smith (Meldrum 32), Kinniburgh (Robert Donnelly 55), McLean, Paul Quinn, Corrigan (McBride 34), McCormack, O'Donnell, Hammell, Paterson, Clarkson, Hamilton. Subs Not Used: Kerr, McGarry, Fitzpatrick, Reynolds.
Booked: Paul Quinn, McBride.
Goals: Hamilton 60.
Dundee Utd: Samson, Duff, Kenneth, McCracken, Gardiner, McIntyre (Goodwillie 73), Cameron, Mulgrew (Easton 72), Robson, Miller, Fernandez (Samuel 64). Subs Not Used: McLean, Robertson, Abbot, Watson.
Goals: Samuel 86.
Ref: S Dougal