The home side were sluggish throughout, especially in the first half when they could have easily have fallen behind as early as the sixth minute.
Winger Chris Sedgwick, who was back after a one-match suspension, shot over the bar from the edge of the penalty area after good work by the recalled Mark Robins.
The Millers took the game to the home side and it was Richard Barker who headed wide of the right-hand post with a decent chance.
Full-back Paul Hurst had a great shot which David Lucas did well to tip over although no corner was awarded.
In a rare threatening moment, Preston went close when Richard Cresswell laid on a chance to Erik Skora, but he shot wide before the visitors took the lead a minute later early in the second half.
An Andy Monkhouse left wing cross fell to Robins, who slid in and the ball deflected in off Chris Lucketti.
The Lilywhites had to respond and David Healy hit an 18 yard shot which forced the Millers keeper Mike Pollitt to tip his effort over for a corner.
With five minutes to go, Rotherham made sure of the points through Richie Barker's mis-hit overhead kick that deceived Lucas and went in off the post.
Afterwards, Preston boss Craig Brown said: “It was not a happy Christmas!“The first goal was a gift but Rotherham are a good, hungry and alert side and played well.â€