Smith settled a tight encounter against AFC Telford United after Jason St Juste and Josh Wilson had earlier traded goals.
Telford's Ben O'Hanlon was handed his marching orders during the closing stages.
The result extended the Pilgrims’ improving league form to just one defeat in nine encounters and moved Dennis Greene’s side to 12th in the National League North standings.
The Pilgrims were indebted to on-loan goalkeeper Dion-Curtis Henry for keeping them in the contest during the opening stages as he saved well to deny Bradley Reid and Jonathan Royle.
Boston threatened for the first time when Callum Chippendale freed Jay Rollins, but he could only fire too high from 25 yards.
The visitors took the lead with 15 minutes played when Chippendale worked the ball across to ST JUSTE who beat James Montgomery with a precise finish into the far corner for his first goal in amber and black.
Chances were traded after the break, with Henry denying Reid again while Waide Fairhurst lashed an effort over the top for Boston.
Telford levelled with 20 minutes of the half played when WILSON fired home the loose ball after initially being denied by a Joe Robinson block on the line.
The home side looked the likelier to snatch all three points but their hopes were hampered when full-back Ben O’Hanlon hauled down the marauding Rollins and received his marching orders. Joe Maguire’s subsequent free-kick was blocked, with Marcus Marshall firing the rebound wide.
Boston snatched the victory in the first minute of stoppage time when Marshall and Rollins combined to tee up SMITH, who gleefully turned home the latter’s cross from close-range.
AFC Telford (3-5-2): James Montgomery; Ben Bailey, Paul McCone (Josh Wilson 60), Gianluca Havern; Dwayne Samuels, Jonathan Royle, Sean Williams, Jack Rea, Ben O’Hanlon; Bradley Reid (David Hibbert 56), Connor McCarthy (Danny Reynolds 82). Subs (not used): Jordan Broome, Zak Lilly.
Boston (4-4-2): Dion-Curtis Henry; Ben Gordon, Joe Robinson, Nat Brown, Courtney Wildin; Marcus Marshall, Callum Chippendale (Kalern Thomas 69), Lewis Hilliard (Joe Maguire 53), Jason St Juste; Jay Rollins, Waide Fairhurst (Gregg Smith 53). Subs (not used): Cameron Johnson, Michael Emery (gk).
Referee: Robert Massey-Ellis.
Attendance: 1,021.