Rushden and Diamonds' five-year stay in the Football League was ended by Boston after two late goals condemned them to a 2-0 defeat and relegation.
Lawrie Dudfield latched on to a Julian Joachim flick and slotted home the opener in the 89th-minute.
And Danny Thomas sealed Boston's win and Rushden's fate in injury time when he drilled home from 18 yards.
The hosts came closest through Wayne Hatswell's header and Brad Maylett long-range shot but it was not enough.
Boston Utd: Logan, Canoville, White, Ellbraith (Melton 21), Till (Dudfield 32), Joachim, Thomas.
Subs not Used: Silk, Chinn.
Goals: Dudfield 89, Thomas 90.
Rushden & D'monds: Tynan, Nicholls, Gulliver, Hatswell, Bull (Tomlin 81), Turner (Berry 53), Caskey, Stokes, Kelly, Broughton (Mikolanda 74), Jackson.
Subs Not Used: Hunter, Crane.
Booked: Gulliver, Jackson.
Rushden are relegated
Ref: C Webster (Tyne & Wear).