Dayle Southwell had fired the Pilgrims into a 2-0 lead inside 50 minutes, only for ex-United duo Spencer Weir-Daley and Greg Mills to haul the Steelmen level.
With the match heading for a stalemate, Garner popped up to bag the decisive goal on 88 minutes and send the travelling fans wild.
Pilgrims' boss Dennis Greene had named an unchanged line-up ahead of kick-off and his selection paid off with a 12th minute opener.
Aaron Greene was fouled around four or five yards outside the penalty area and SOUTHWELL stepped up to fire home a low free-kick which nestled into the net off the inside of Paul Walker's far post.
The Pilgrims were full of running during the first half and played some good football on a surface quickened up by the torrential rain which lashed the ground throughout.
Southwell and Mark Jones had further efforts for the Pilgrims, but neither side registered a shot on target during the reminder of the first half.
United doubled their lead five minutes after the break and moments after Weir-Daley had nodded narrowly over from an Ashley Sammons free-kick.
Garner helped the ball back into the danger zone and SOUTHWELL arrowed a 20-yard beauty beyond the Corby keeper and into the top corner.
United's two-goal buffer was shortlived though and WEIR-DALEY soon glanced home Sammons' inviting corner delivery.
Corby levelled on 66 minutes when Henrich Ravas hauled down Kabongo Tshimanga in the penalty area and MILLS stroked home the resulting spot-kick.
The match seemed to be heading for a draw until substitute Nicky Walker blasted goalwards with two minutes to play and when namesake Paul Walker was unable to hold on, GARNER gleefully rammed home the loose ball.
Corby (4-2-3-1): Paul Walker; Kalern Thomas, Duane Courtney, Cedric Kipre, Chris Carruthers; Cleveland Taylor, Ben Milnes; Kabongo Tshimanga (Kurtis Revan 76), Ashley Sammons, Greg Mills; Spencer Weir-Daley. Subs (not used): Connor Kennedy, Danny Draper, Michael Ogilvie, Sam Donkin (gk).
Boston (4-4-2): Henrich Ravas; Jordan Richards, Zak Mills, Carl Piergianni, Liam Marrs; Jay Rollins (Jay Dowie 76), Scott Garner, Kyle Dixon, Aaron Greene (Nicky Walker 68); Dayle Southwell, Mark Jones (Lartey Sarpong 53). Subs (not used): Jamie McGhee, Jordan Deacey.
Referee: Amy Fearn.
Attendance: 613.