Strikers Billy Clarke and Gary Alexander scored either side of the break and Pablo Mills' close-range header sealed a 3-0 win for Steve Evans' men.
The Millers had their moments in the first half and Gareth Evans was thwarted by Kyle McFadzean following a cross from Mark Bradley, before Alex Revell fired against the crossbar after being set up by Kieron Cadogan.
But Crawley broke the deadlock two minutes before the interval when Alexander surged down the right and crossed low and hard for strike partner Clarke to steer home his second goal for the club from close range.
Alexander, on loan from Brentford, doubled Crawley's advantage from the penalty spot eight minutes after the interval when Jason Taylor was penalised for a push on Dannie Bulman.
Rotherham goalkeeper Rhys Taylor produced a fine double save to deny Sergio Torres and McFadzean before Town skipper and ex-Miller Mills made it 3-0 with a far-post header following a corner from Scott Davies.
Revell again went close at the oher end with a misplaced header and sub Lewis Grabban blasted over, but it wasn't to be the Millers afternoon.
Source: DSG