Folan has attracted interest from Blackburn Rovers amongst others, and did his cause no harm with his 13th goal of the season in the 33rd minute.
Both sides set about each other eagerly in this local derby. Chesterfield went close early on when Folan had a smart header tipped over the bar by Neil Cutler from Adam Smith's cross.
Then Rotherham's latest signing Ian Henderson, picked up on loan from Norwich, raced onto a long ball forward and unleashed a fierce drive from 20 yards which just went over.
Chesterfield's own loan capture, Peter Holmes from Luton, received a pass from Paul Shaw, jinked inside two players and forced Cutler into another good save.
Chesterfield continued to press and on 17 minutes, Folan whipped in a low cross which Shaw just failed to get on the end of.
The Spireites were playing against the wind but were creating the better chances against a Rotherham side playing their first game since losing top goal scorer Will Hoskins and star midfielder Lee Williamson to Watford.
Holmes stabbed a shot wide on 27 minutes, but on 33 the visitors got a deserved breakthrough.
Mark Allott's long throw was not dealt with by the Rotherham defence. Shaw, with his back to goal, held up the ball and teed up Folan who fired into the bottom corner from 12 yards.
Chesterfield almost doubled their lead straight after the restart when Aaron Downes climbed above a static defence but he powered his header wide.
There was still life left in Rotherham and again it was new man Henderson going close, as he headed Stephen Brogan's cross wide.
Midway through the half Rotherham fashioned a chance when David Worrell was released down the right, and his cross was volleyed over by substitute Marc Newsham.
Shaw then charged up the other end and dragged a shot across the face of goal. Holmes then curled a 30-yard free-kick over the bar 14 minutes from time.