Josh Hmami struck the only goal of the game in the eighth minute as FC Halifax Town chalked up their first victory of the season.
United’s best chance fell to Lenell John-Lewis before half-time, but he was denied by Shaymen keeper Sam Johnson.
A lightning break down the Halifax left involving Thierry Latty-Fairweather did the trick for the hosts' breakthrough, with HMAMI steering home an effort at the near post to break the deadlock.
John-Lewis forced Johnson into a smart save in response, while the Pilgrims soon lost Jordan Cropper to a hamstring complaint. John-Lewis then headed a cross from Cropper's replacement, Rocco Coyle, over the bar.
Johnson kept out another John-Lewis effort a minute before half-time.
After the break, John-Lewis was inches away from converting an Alex Lankshear cross from the left - but the Pilgrims struggled to make serious inroads against the towering Halifax backline which had lost Jevon Mills to a shoulder injury in the first half.
Boston (3-4-1-2): Rhys Lovett; Ben Grist, Tommy Fogarty, Connor Teale; Jordan Cropper (Rocco Coyle 31), Jordan Richards, Greg Sloggett, Alex Lankshear; Dylan Hill; Lenell John-Lewis (Manni Norkett 81), Kieren Donnelly (Jordy Hiwula 72). Subs (not used): Arjan Raikhy, Finley Ging, Billy Bradley, Bobby Jones (gk).
FC Halifax (4-2-3-1): Sam Johnson; Sean Tarima, Shaun Hobson, Jevon Mills (Adam Adetoro 28), Thierry Latty-Fairweather; Will Hugill, Cody Johnson; David Kawa (Angelo Cappello 69), Josh Hmami, Owen Bray (Jay Turner-Cooke 81); Jamie Cooke. Subs (not used): Jack Jenkins, Tom Pugh, Zak Emmerson, Nat Ford (gk).
Referee: Callum Walchester.
Attendance: 2,205 (95 visitors).
PIC: Chris Bray.