Second-half goals from striker Kion Etete and full-back Perry Ng were enough for the sixth-placed Bluebirds to claim all three points and continue their recent surge up the table.
It was not an encounter that will live long in the memory, but Erol Bulut's home side were full value for the victory, with a dismal Rotherham team barely laying a glove on them all afternoon.
The first half could scarcely have been a drabber spectacle.
Both sides managed some neat and intricate passing, but largely in the middle third.
Cardiff enjoyed most of the territory and possession, but did very little with it.
Their best opportunity before the interval fell to captain Ryan Wintle who found space in the box in the 43rd minute, only for Rotherham goalkeeper Viktor Johansson to comfortably save his tame effort.
Before that, a scuffed attempt from right wing Yakou Meite and Etete's speculative 20-yard shot was the closest the home side came to breaking the deadlock.
The Millers were even less threatening, ending the first half without creating a single opportunity worthy of note.
Their bluntness spoke volumes about their disappointing start to the campaign, but Cardiff fans will have expected more from the hosts.
Any momentum from the recent upturn in fortunes was nowhere to be seen, with the Bluebirds lacking urgency and attacking ideas.
With the second half came a marked increase in tempo from Cardiff - and they were rewarded with the opening goal in the 56th minute.
Left wing Karlan Grant exchanged passes with Joe Ralls on the flank and Grant's cross was met by a downward header into the bottom corner from Etete, who outmuscled Rotherham full-back Cohen Bramall far too easily.
Cardiff's dominance of the ball continued. Substitute Ollie Tanner - who replaced the injured Meite in the 18th minute - flashed a shot just wide, and Grant increasingly made his presence felt out wide.
Rotherham waited until the 80th minute to threaten an equaliser when substitute Arvin Appiah cut in from the right and sent a left-footed shot just over the crossbar.
It triggered a promising spell of pressure from the away side, but fashioning clear-cut chances remained a problem.
Cardiff, with centre-half Mark McGuinness outstanding, were rock solid at the back.
And just a minute before the end of normal time the Bluebirds counter-attacked for a second goal.
Ng rammed home a loose ball in the box after Rotherham botched an attempted clearance and the win was sealed.