And it was no more then Ronnie Moore's men deserved as they collected their second victory of the Bank Holiday weekend and soared into the upper reaches of the first division.
But it was a miserable weekend for John Gregory's Derby side who were hammered on Saturday by Wolves.
Rotherham boss Moore was delighted with his side and said: "I thought we were magnificent, we were always the better side and deserved the win.
"It's been a great weekend for us. For me the win over Sheffield Wednesday was better but we have done tremendously well against a team who have just come down from the Premiership.
"But we are realistic and we won't get carried away. We're still favourites to go down and our target is still 50 points to avoid relegation. Our fans are enjoying it. They think we're in second spot so it's automatic promotion and who knows?"
Derby were always second best after Rotherham, still bouyant from their victory over local rivals Sheffield Wednesday, set a blistering early pace.
Rotherham's pressure was such that they picked up an amazing ten first-half corners. They forced Derby on to the back foot.
And keeper Andrew Oakes had to be at his best as the home side took control.
Oakes had to fingertip over the top early on as Derby's Rob Lee accidently diverted a corner goalwards.
But despite the pressure Rotherham were unable to make it count and it was Derby who took the lead immediately after the restart.
Branko Strupar stooped to head in a free-kick from Lee that glanced of the head of Rotherham defender Rob Scott.
But in the 65th minute Rotherham levelled. Lee was hovering to finish off from close range after John Mullin's shot was blocked.
Then eight minutes from the end, Martin McIntosh headed Darren Garner's free kick and Lee controlled superbly on his chest before swivelling and driving his shot passed Oakes