County failed to build on their last three, much-improved performances and crashed to a bitterly disappointing defeat against their relegation-threatened rivals.
Defensively, the Hatters again looked solid, but in midfield Dons always had the edge largely through their physical presence and commitment.
Consequently the flow of the game was mainly in the direction of James Spencer's goal but, apart from Ben Harding's long-range drive on 19 minutes, the young 'keeper had little to trouble him in the opening exchanges.
That all changed just after the half-hour, though, when the over-lapping Leon Crooks crossed from the left and Spencer could only push the ball into the path of Dean Lewington who scored from 10 yards.
County almost equalised 2 minutes before the break when debutant Jordan Hadfield skipped past Crooks and crossed to Kevan Hurst at the near post. The Sheffield United man turned smartly and was unlucky to see his shot deflected for a corner.
Dons started the second half in determined mood and in the opening 5 minutes they almost scored twice. Gareth Edds drove a shot wide from the edge of the box then Lewington brought a great save out of Spencer.
The pressure paid off on 59 minutes when a clearance found the unmarked Edds 20-yards from goal and this time he got his direction right to beat Spencer just inside his right-hand post.
County gave themselves a lifeline with a goal 17 minutes from time. Hurst floated the ball in from the left, which Matt Baker failed to gather after colliding with Chris Armstrong and Keith Briggs scored at the second attempt.
A barnstorming finish, though, unfortunately never materialised and County's cause wasn't helped when John Hardiker was dismissed 6 minutes later after receiving a second yellow card for a tackle from behind on Lewington.
In the end the Dons held on comfortably to win the three points that moved them out of the bottom four for the first time since September.
MK Dons: Baker; Oyedele, Chorley, Pensee-Bilong, Crooks, Lewington, Harding, Edds, Small, McLeod (Puncheon 88), Platt.
Unused Subs: Kamara, Smart, Palmer, Rizzo
Goals: Lewington 34, Edds 59
Yellow Cards: Chorley 40, McLeod 87, Puncheon 90
County: Spencer; Goodwin, Dolan, A Williams (Raynes 77), Hardiker; Briggs, Hadfield (Le Fondre 55), Robertson, Hurst; Armstrong (Jackman 82), Feeney.
Unused Subs: Cutler, Barlow
Goals: Briggs 73
Yellow Cards: Hardiker 41 & 79, Robertson 90, Goodwin 90
