The Pilgrims emerged from a tricky trip to Nuneaton Town’s Liberty Way with a 1-1 draw to seal the title ahead of nearest challengers Basford United.
Withers struck on the half-hour mark and although Boro levelled eight minutes from time, the depleted Pilgrims held on for a title-clinching share of the spoils.
Boston (4-3-3): Harry Payne; Jordan Tate, James Gordon, Jack Withers, Jack Brown; Liam Adams, Peter Evison, George Hobbins; Arkadiusz Michalczyk, Maison North (Connor Stocks 75), Haydn Havard. Sub (not used): Seth Greeves (gk).