A second-half strike by Benik Afobe gave MK Dons a hard-earned 1-0 win at the Proact Stadium and ended Chesterfield's unbeaten start to the season.
MK Dons started confidently with Ben Reeves slicing wide from eight yards but Chesterfield had a great chance in the 29th minute when Sam Hird headed a Daniel Jones cross wide.
Gary Roberts and Kyle McFadzean were booked for an off-the-ball clash after Roberts went in late on'keeper Dave Martin, who did not come out for the second half.
His replacement, Ian McLoughlin, should have been beaten in the 46th minute but Eoin Doyle fired wide from 12 yards.
The visitors were ahead three minutes later when Afobe cut in from the right and beat two defenders before driving a shot inside Tommy Lee's near post.
Afobe missed two chances to seal it and Chesterfield pressed hard with Ian Evatt heading over in stoppage time, but MK Dons held on.