Mark Hudson helped Rotherham to their seventh league win in a row at the expense of his former club.
Derek Holmes had given Rotherham an early lead after goalkeeper Barry Roche had spilled a deflected shot from forward Chris O'Grady.
Hudson, who was booed by the home fans during the match, added the second in the 55th minute.
Steve Fletcher came close to pulling one back for Chesterfield who have not won at home since early September.
Chesterfield: Roche, Lowry, Kovacs, Gray, Robertson, Bastians (Picken 64), Leven, Niven, Rooney (Smith 64), Fletcher, Ward.
Subs Not Used: Downes, O'Hare, Allison.
Booked: Niven, Fletcher, Smith.
Rotherham: Warrington, Joseph, Coughlan, Sharps, Brogan, Hudson, Mills, Danny Harrison, Derek Holmes (Tonge 82), O'Grady, Taylor (Peter Holmes 36).
Subs Not Used: Annerson, Hurst, Newsham.
Booked: Peter Holmes, Sharps.
Goals: Derek Holmes 8, Hudson 55.
Att: 5,417
Ref: Mike Russell (Hertfordshire).
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