Crawley reignited their promotion hunt with a third-successive win as managerless Rotherham were swept aside.
Gareth Evans went close and Alex Revell struck the crossbar for the visitors in the first half, but that was as good as it got.
Gary Alexander's low cross was steered home by Billy Clarke, before Alexander made it 2-0 from the spot after Jason Taylor pushed Dannie Bulman.
Pablo Mills added a third with a far-post header from Scott Davies' corner.
Crawley are now three points behind third-placed Shrewsbury in fourth, and have a game in hand.
Rotherham stay in 10th place, but are now five points adrift of Gillingham in ninth.
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Crawley Town boss Steve Evans told BBC Sussex:
"I thought the first half was as poor as we'd been for a little while. We went in at half-time and had to rattle one or two cages. They went flying in the second half.
"We over-ran them and Rotherham went home fortunate it was only three. Good strikers win matches. It's a good victory for us and we can only think about ourselves.
"We had some big players missing. Every week is going to be a big game and every point or three is going to be vital."
Rotherham United caretaker manager Darren Patterson told BBC Radio Sheffield:
"I am obviously disappointed with the result, but first half we should be comfortably three or four in front.
"We absolutely battered them, couldn't convert our chances and get done with a sucker punch. I am disappointed second half we conceded such a sloppy goal early on.
"We've come here to win, so we've had a little gamble, put three centre-forwards up front to have a go and we conceded another soft goal."