Johnny Russell netted the only goal of the game after 17 minutes and despite piling on the pressure during the second-half the hosts were unable to come up with a response.
McFadden - who will be with the club for the rest of the season - made a late appearance from the bench, however he could not prevent Motherwell falling to their first home defeat of 2013.
Stuart McCall's side made a bright start and had two good opportunities to take advantage of defensive lapses from United in the opening two minutes.
Firstly Sean Dillon misplaced a back-pass which Henrik Ojamaa intercepted before driving forward and winning a corner, then Brian McLean hooked a clearance wide of his own-goal.
Chris Humphrey's low drive drifted wide of Radoslaw Cierzniak's left-hand post as the home side continued to control proceedings and Nicky Law came close to opening the scoring when he linked well with Ojamma after 16 minutes.
The home side were to be hit on the break just one minute later however, when Gary MacKay-Steven burst forward before slipping the ball through for Russell who showed excellent composure in firing the ball past Darren Randolph.
Russell came close to doubling his tally after pouncing to take advantage of some slack defending by Steven Hammell, however this time he sent an effort just over the top.
Motherwell continued to press as the first-half went on, but found clear-cut chances at a premium.
Shaun Hutchinson was the first player to receive a booking after a careless challenge on MacKay-Steven and from the resulting free-kick McLean fired over the top following Russell's knock-down.
Ojamaa blazed high over the crossbar at the hosts responded at the other end and they were presented with a good opportunity to grab a leveller when Tom Hateley was felled at the edge of the box by Dillon.
Kallum Higginbotham stepped up to take the resulting free-kick, but was unable to test Cierzniak.
After spending the majority of the opening 20 minutes of the second-half on the back-foot, United spurned two good chances to extend their lead within the space of a minute.
MacKay-Steven forced Randolph into a good stop with his feet and then Jon Daly headed over the bar from the edge of the six-yard box.
Simon Ramsden called Cierzniak into action with a low shot on 70 minutes, but again it was United who almost profited on the break - MacKay-Steven twice bringing out superb stops from Randolph and then passing up a glorious chance when he failed to convert after the Republic of Ireland stopper denied Richie Ryan.
The Motherwell supporters roared in approval as McFadden made his first appearance since re-joining the club when he replaced Ojamaa and Bob McHugh also came on in place of Higginbotham.
The introduction of the former Everton man sparked life into the home crowd and they were almost rewarded when Law floated a shot just over the crossbar.
MacKay-Steven had yet another chance to put the game out sight when he brilliantly controlled Stuart Armstrong's pass and this time beat Randolph, however Hutchinson made an excellent goal-line stop on the line.
Jackie McNamara made a late switch when he brought on Mark Millar for Willo Flood and the Tannadice side were to hold on for all three points, a victory that puts them level on points with Kilmarnock in fifth place.