His 34th-minute strike at East End Park took Stephen Kenny's side to within a point of second-bottom St Mirren.
Dunfermline fully merited the three points and they should have won by a more comfortable margin.
McManus sent an early header straight at Derek Stillie before Jim McIntyre scooped a six-yard shot wide after McManus had set him up.
Barry Robson gave the home side a scare with a 25-yard free-kick which slipped just inches past the left-hand post.
But the game turned within a few first-half seconds. Christian Kalvenes saw a shot blocked by Dorus de Vries and Dunfermline raced to the other end to score the decisive goal.
With Kalvenes out of position, Greg Shields swept over a cross and McManus crashed home a right-foot volley.
Dundee United had the lion's share of the possession in the second half but it was Dunfermline who looked the more likely to add to their lead.
Stephen Glass should have given them a two-goal cushion when McManus sent him clear but, after flicking the ball past Stillie, he then hooked his shot over the top in front of an open goal.
United pressed for the equaliser and Dave McCracken sent a close-range header wide but it was the home side who held on for an invaluable three points.