The Millers started on the front foot and Aldershot looked in danger of being overrun as the home side carved out four chances in the first ten minutes. Ben Pringle shot over, Michael O'Connor fired wide, Kieran Agard had an effort blocked and Alex Revell's radar was just off.
But with a determined rearguard action Aldershot weathered the storm and in the 27th minute Michael Rankine had their first attempt on goal with a header that landed just over the bar. And six minutes later Asa Hall failed to test the goalkeeper when he met Craig Stanley's corner.
Revell then went close for Rotherham with a drive which was saved by Jamie Young and the home side had another chance from Pringle's free kick but no one could turn the ball home.
And a minute before the break Aldershot almost took a shock lead when Rankine got a foot to a Danny Rose cross but he turned the ball wide of the target.
The Shots pushed forward much more after the break, but Rotherham finally got the breakthrough they needed in the 64th minute. O'Connor's free kick was fumbled by Young and in a goalmouth scramble it was skipper Johnny Mullins who turned the ball home from close range for his fourth goal of the season.
And in injury time Lee Frecklington made absolutely certain of promotion when Pringle broke away to set up an unmissable chance.