Jack Cowgill was red-carded in the incident which saw Reece Styche open the scoring from the spot with less than a quarter of the contest played.
Kabongo Tshimanga equalised two minutes after half-time but Darryl Knights won it for the hosts with a spectacular strike in the 71st minute.
United battled well with 10 men, but ultimately came up short in their second away contest of the season.
Tamworth’s Aman Verma had an early effort deflected wide before the game’s defining moment unfolded. Cowgill shoved STYCHE as he ran into the box and once the red card was hoisted, the Tamworth striker beat George Willis with a spot-kick which bobbled home.
The Pilgrims responded with Mason Warren testing Shaun Rowley from distance, while Tom Batchelor headed wide following Ashley Hemmings’ free-kick.
United levelled just over 60 seconds after the restart. Hemmings threaded substitute TSHIMANGA into the clear and the former MK Dons frontman beat Rowley from 18 yards.
Styche headed wide from a Stephan Morley free-kick as Tamworth threatened to exploit their numerical advantage and they did regain their lead when KNIGHTS lashed home a fine effort from 18 yards after dissecting the United defence.
There were late chances for Morley of Tamworth, with Willis saving well, and United sub Taron Hare, but there were no further goals.
Tamworth (3-5-2): Shaun Rowley; Louis Ezewele, Luke Jones, Regan Upton; Andrew Burns, Aman Verma, Ellis Deeney, Darryl Knights (Jed Davies 80), Stephan Morley; Reece Styche, Connor Taylor (Bradley Reid 75). Subs (not used): Matt Curley, Jack Lane, Brandon Fox.
Boston (3-4-3): George Willis; Tom Batchelor, Jack Cowgill, Jordan Keane; James Clifton, Adam Chapman, Jack Broadhead, Brad Beatson; Ashley Hemmings (Tyrell Waite 75), Karl Hawley (Kabongo Tshimanga 46), Mason Warren (Taron Hare 57). Subs (not used): Harry Vince, Jamie McGuire.
Referee: Amy Fearn.
Attendance: 702.