Gustav Lindgren's ninth minute strike was the difference between the sides at a sodden Jakemans Community Stadium on Saturday afternoon.
United's best chances fell to Kieren Donnelly and Frankie Maguire, denied by Alex Bass either side of half-time, while the Pilgrims' trialist striker rattled the post late on.
United: Bobby Jones, Michael Kelly (Billy Bradley 67), Connor Teale (Ben Grist 67), Zak Mills (Jacob Scott 67), Greg Sloggett (Olly Green 67), Rocco Coyle (Jordan Cropper 67), Dylan Hill (Trialist C 67), Frankie Maguire (Liam Waldock 67), Kieren Donnelly (Manni Norkett 46), Alex Lankshear (Finley Ging 67), Jordan Richards (Arjan Raikhy 67). Sub (not used): Rhys Lovett (gk).
Referee: Zac Kennard-Kettle.
Attendance: 1,505 (482 visitors).
PIC: Chris Bray.