Millers 3 Chesterfield 0
It was a bitter setback for Lee Richardson's side who saw their seven match unbeaten run come to an end at the hands of their near neighbours.
And Richardson's side were left to survive the last 11 minutes with ten men after Drew Talbot was sent off for a second yellow card.
Jamie Lowry menaced for Chesterfield down the right and carved out chances for leading scorer Jack Lester and strike partner Martin Gritton.
Lowry cut in an early ball but a tame shot from Lester failed to trouble Andy Warrington, soon after a 12th minute ball inside found Gritton in space and with only Warrington to beat he crashed his shot wide.
And when Jared Wilson provided a clever pass for Lester the 17-goal hitman was denied another when his angled shot was well saved by Warrington.
But the home side caught out their local rivals with a crisp move involving Mark Hudson and Micky Cummins to provide Reid with an opportunity after 39 minutes and Reid angled in a low shot that gave Tommy Lee no chance with his 14th goal of the season.
Seconds before the end of the first half Lewis Montroswe came close to an equaliser when he twisted inside from the right but his shot from 18 yards drifted over.
Rotherham stretched their lead ten minutes into the second half when a long ball from Pablo Mills was flicked on by Drewe Broughton to Reid and he provided the coolest of finishes when he lifted a deft ball over Lee .
Former Rotherham defender Gregor Robertson went close to pulling one back but Warrington did well to save his free-kick from 25 yards
Rotherham skipper Pablo Mills rounded off the scoring when he slammed in a shot from 20 yards.