Jordon Crawford handed the Pilgrims an eighth minute lead - only for Reece Kendall and Glen Taylor to turn the tables.
Jo Cummings equalised against his old club with 13 minutes to play, but Joe Tait won it for the hosts a minute later with a long-range thunderbolt.
United evening had got off to the perfect start, with CRAWFORD bundling the ball home from close-range after home keeper Gio Bellagambi had made a hash of punching clear Scott Pollock's free-kick delivery.
Spennymoor's first real spell of pressure brought chances, with Rob Ramshaw and Taylor denied by a combination of Sam Long and the post.
United were inches away from doubling their lead after the break, with Scott Pollock and Alex Brown both agonisingly close to converting Keenan Ferguson's cross from the right.
The Moors levelled in the 58th minute when Brown allowed John Lufudu too much time to deliver from the Town right and KENDALL climbed above Ferguson to head home at the back post.
The home side took the lead a minute later when Ramshaw delivered from the left and TAYLOR volleyed home a smart finish from 12 yards.
Bellagambi produced a sensational save to deny Crawford as United threatened an equaliser - and their leveller did arrive with 13 minutes to play when CUMMINGS slammed the ball home after seeing his initial effort saved by Bellagambi from a Scott Pollock free-kick.
Parity was short-lived, with TAIT advancing to whack a 30-yarder beyond Long's grasp.
United thought redemption had arrived in stoppage time, but sub Jake Wright turned and dragged an effort narrowly wide.
Spennymoor (3-4-1-2): Gio Bellagambi; Joe Tait, James Curtis, Michael Ledger (Finn Cousin-Dawson 82); John Lufudu, Dylan Boyle, Elliott Whitehouse, Reece Kendall; Rob Ramshaw; Glen Taylor, Tom Allan (Paul Blackett 70). Subs (not used): Corey McKeown, Lewis Thompson, Harry Flatters (gk).
Boston (3-5-1-1): Sam Long; Jo Cummings, Shaun Pearson, Ben Pollock; Keenan Ferguson (Joe Starbuck 70), Lebrun Mbeka, Tom Platt, Will Atkinson (Jordan Burrow 73), Alex Brown; Scott Pollock; Jordon Crawford (Jake Wright 73). Subs (not used): Tom Solanke, Jake Lovelace (gk).
Referee: James Bancroft.
Attendance: 1,125 (35 visitors).
PIC: Ian Kelsey.