Dons dealt biggest league defeat of the season by Rotherham
The warning signs were there early on when Michael Smith hit the crossbar within the first five minutes, and it came as little shock when Michael Ihiekwe headed Rotherham in front after 27 minutes.
Dons struggled to create chances, though Peter Kioso and Zak Jules came close to levelling in the first half, but when Dan Barlaser scored direct from a corner on 66 minutes for the visitors, the writing was on the wall.
Freddie Ladapo added a third with 18 minutes to go to complete the rout as Dons dropped to seventh in the standings.
Zak Jules came into the side with Daniel Harvie suspended after picking up his fifth booking of the season on Tuesday night against Wigan. Will Grigg, who scored eight goals for Dons last season on loan from Sunderland, was in the line-up for Rotherham.
The Millers were eight unbeaten heading to Stadium MK and looked on the crest of the wave in the opening 45 minutes, and though eyes were on Grigg, it was his strike partner Michael Smith who caused the biggest problems for the visitors early on as he clipped the crossbar early, trying to lob Andrew Fisher in the Dons net.
With both sides fancying their chances of getting an opener, Peter Kioso and Scott Twine both came close with efforts but couldn't force Josh Vickers into a save, while Rotherham looked a significant threat with some unique set-piece routines.
And it was with one of their more conventional set-pieces which saw them take the lead on 28 minutes. Launched into the penalty area, Fisher didn't know whether to come or stay, and skipper Michael Ihiekwe rose highest to nod goalwards as it sailed over the Dons keeper to give the visitors the lead.
It should have been 2-0 five minutes later when Jules gave away possession in the centre of the park and that man Smith again broke clear, but his touch at the defining moment was heavy and allowed Fisher to gather at his feet as the striker shaped to shoot.
Though looking second best for the majority of the half, Dons could have been level with a couple of chances when David Kasumu's volley was deflected just over, and then in first half stoppage time when Harry Darling headed wide, and though Twine was able to recycle the ball back into the mix, Jules powered his header over the bar.
The second half didn't get much better for Dons, despite a bright start to the half. Chances were still hard to come by despite having the better of the opening exchanges, but once more it was a Rotherham set-piece which would put the result beyond their reach, as Dan Barlaser scores direct from a corner on 66 minutes to make it 2-0.
Almost to rub salt in the wounds, substitute Mickel Miller had only just come on before skipping past the challenge of Matt O'Riley, pulling back to another substitute Freddie Ladapo to thrash home a third for Rotherham with 18 minutes to go.
Referee: Chris Sarginson
MK Dons: Fisher, Lewington, OHora, Darling, Jules, Kioso, Kasumu, O’Riley, Twine, Boateng 9Parrott 64), Eisa
Subs not used: Ravizzoli, Watson, Baldwin, McEachran, Watters
Rotherham United: Vickers, Harding, Barlaser (Odoffin 82), Wiles, Grigg (Ladapo 63), Ogbene, Ferguson (Miller 71), Rathbone, Ihiekwe, Edmonds-Green, Smith
Subs not used: Jonansson, Wood, Sadlier, Kayode
Booked: Rathbone, Kasumu, Ogbene