The former Swindon striker met Billy Jones' cross one minute into added time to move the Robins back back into the top three.
A Jones' corner created the first chance of the match which Keith Lowe nodded over the bar in the sixth minute as Cheltenham made the stronger start.
Chesterfield improved as the first half wore on and Jay O'Shea saw a firm drive blocked by Steve Elliott in the 18th minute.
Drew Talbot also saw a shot hit Elliott and deflect behind for a corner after Conor Townsend reached a cross form Sam Togwell, but his header was wide of the near post as the first half ended goalless.
It was a scrappy affair, with neither side able to find the killer ball, but substitute Jermaine McGlashan made a positive impact for Cheltenham on the right wing.
They looked the more likely winners, with Jason Taylor going close with a shot from the edge of the box that just cleared the bar before Benson settled it.