It was a tough afternoon for the Millers, who were brought back down to earth with a bump after wins over Northampton Town and Exeter City.
Cardiff played with 10 men for the bulk of the game after Ryan Wintle was dismissed, yet still looked incredibly comfortable.
Goals from Omari Kellyman, Chris Willock and Isaak Davies put Rotherham to the sword and the hosts were perhaps fortunate it was not an even heavier defeat.
Here are The Yorkshire Post’s player ratings from the New York Stadium.
1. Rotherham United player ratings v Cardiff City
The ratings are in following Rotherham United's meeting with Cardiff City. | Pete Norton/Getty Images
2. Cameron Dawson - 6
Not to blame for the lack of inspiration in front of him and produced some decent saves. | Pete Norton/Getty Images
3. Joe Rafferty - 6
Full of energy but he did not appear to have the skillset to cause Cardiff any real problems as a wing-back. Substituted at half-time. | Pete Norton/Getty Images
4. Zak Jules - 5
Never really looked comfortable playing out and had a mixed afternoon in his battles. | Jonathan Gawthorpe