The victory, earned by a goal in each half from Joe Newell and Semi Ajayi, was the Millers’ fourth win in succession and they are now unbeaten in 11 games.
Rotherham had always looked the more likely scorers before winger Newell hooked them in front in the 31st minute on the back of the home side’s failure to deal with a long throw into the penalty area.
Although the Iron upped their game after the goal, the Millers survived a penalty shout involving keeper Mark Rodak and Ivan Toney and a decent opening in a crowded box from which Rory McArdle was off target.
After the break, having weathered a Scunthorpe storm, the visitors were rewarded with a second goal when centre-half Ajayi looped home a header after a game of head tennis.
And though substitute Lee Novak responded almost immediately by rounding Rodak and slotting home, the Millers closed the game out comfortably to move within a point of their hosts.