Top three Rochdale enjoyed the better of the first half in difficult windy conditions at Spotland, but it was the visitors who created the first half's only decent opportunity when the ball fell to Ryan Taylor in the box, but his effort on the turn was well saved by home keeper Frank Fielding.
Now safe from relegation with their points deduction wiped out and with a faint chance still of making the play-offs the Millers enjoyed much the better of the second half and took the lead within five minutes of the restart.
A Dale Tonge free-kick landed in the home penalty area and was brought under control by leading scorer Reuben Reid who held off two challenges before turning and shooting smartly past Fielding.
Home manager Keith Hill immediately made a double substitution in an attempt to get his side back into the game, but Rotherham doubled their lead on 68 minutes as Ryan Taylor was allowed to cut in from the right before producing a curling left-footed effort beyond the despairing dive of Fielding.
The home side were given a glimmer of hope seven minutes later when one of the substitutes, Nicky Adams, crossed for leading scorer Adam Le Fondre headed home from close range for his 18th goal of the season.
Rotherham should have made the points safe five minutes later when Ryan Taylor was upended in the box by Rory McArdle, but Reid blasted his spot-kick against the crossbar.
Rochdale huffed and puffed for a late equaliser, but in truth Millers keeper Andy Warrington was never really called into action with Nick Fenton in particular outstanding in the centre of defence.