American striker Dom Dwyer popped up with a 90th-minute winner for Mansfield with his first touch of the ball as struggling Rotherham were beaten 2-1.
Jordan Hugill gave the visitors an early lead from the spot but second-half goals from Tyler Roberts and Dwyer consigned the Millers to a third successive defeat.
A Contested Opener
The Stags were furious when handball was given in the seventh minute as Frazer Blake-Tracy blocked a close-range Hugill shot.