County should have led when Kwesi Appiah just failed to get a foot to Jamal Campbell-Ryce's cross after five minutes but then Rotherham's twin Newcastle loanees took a hand.
Kieran Agard was felled on the edge of the penalty area and James Tavernier curled his 12th-minute free-kick into the top corner.
Four minutes later it was Haris Vuckic's turn. Ben Pringle fed Tavernier and, from his cross, Vuckic saw his shot take a wicked deflection to wrongfoot Bartosz Bialkowski.
But much worse was to follow for County. Pringle won back a ball he had lost and laid off a pass to Alex Revell, who hit a first-time shot hard and low into the net.
And merely minutes later Rotherham went four-nil up from the penalty spot.
Gareth Roberts tried to let the ball run back to his goalkeeper but it was never going to reach and Agard nipped in before being upended by Bialkowski. Agard duly dusted himself down and slammed in his 17th of the season from the spot.
Fifteen minutes after the break Rotherham made it five when Vuckic added is second of the afternoon after a storming run down the right.
And seven minutes from the end a Nicky Adams cross was turned in by Tom Hitchcock for the sixth.