Motherwell left Tannadice with all three points as Dundee United were consigned to a fourth Scottish Premier League defeat this season.
Keith Lasley's strike put the visitors in front in the 14th minute.
An error by United goalkeeper Dusan Pernis led to Michael Higdon firing the visitors' second before the break.
Higdon's penalty, awarded after a trip by Garry Kenneth on Nicky Law, put Motherwell further ahead and Jon Daly struck a late United consolation.
The win moves Stuart McCall's side up to second in the table, a point ahead of Celtic who play Hearts on Sunday.
Jon Daly had a couple of efforts early on, heading over before forcing Darren Randolph into a save.
But it was Motherwell who opened the scoring after an excellent passing move.
Law and Chris Humphrey moved the ball wide to Tom Hateley, who crossed for Lasley to fire high into the net.
Motherwell were comfortable with their lead as United struggled to force their way back into the match.
Law's long-range effort drifted wide and Humphrey's shot was deflected out for a corner before the Tangerines manager decided to change things by introducing substitute Lauri Dalla Valle in place of Ryan Dow.
Dalla Valle glanced a header into the arms of Randolph as the hosts tried to find some momentum but Pernis's mistake in first-half injury time allowed Motherwell to double their lead.
The goalkeeper's wayward clearance presented Higdon with a shooting chance and the striker's low drive found the net via Pernis's last-ditch save attempt and the post.
United tried to rally after the break but could not find a way through.
And Motherwell made sure of their win after Law was tripped in the box by Kenneth and referee Steven McLean pointed to the spot.
Higdon confidently stroked the ball past Pernis from 12 yards in the 64th minute.
Gavin Gunning tried to reduce Dundee United's deficit with a fierce strike but Randolph made a fine save.
United netted when Daly converted a rebound off Randolph's save from Keith Watson's shot.
But that was all the home fans had to celebrate as their side go into the international break in the bottom half of the table.