Fourth in the table MK Dons, who led through an early Clive Platt header, paid dearly for their second-half indiscipline and ended the match with ten men following the 56th-minute dismissal of Jude Stirling - with seven players also finding their way into the notebook of referee Miller.
After Platt put the visitors ahead, MK Dons went on to dominate the remainder of an often scrappy first-half during which Martin Allen's side had a number of chances to settle the issue.
But nine minutes after the re-start, man-of-the-match Derek Asamoah levelled in bizarre circumstances and, from that point, MK Dons fell into disarray.
Stirling was ordered off for an elbow offence involving Shrewsbury midfielder Stewart Drummond - with their misery completed when Ashton netted with a quarter of an hour remaining to secure the points for the Gay Meadow side.
Shrewsbury, unchanged from the side that picked up a precious point at Wycombe on Saturday, were quickly forced on to the defensive by a combative and highly competitive Milton Keynes outfit - who went ahead on 12 minutes when Platt rose to glance home a long throw, hoisted into the Shrewsbury box by Stirling.
Stirling and skipper Keith Andrews then wasted fine opportunities to put the visitors further ahead - chances they would regret not taking as the second-half unfolded.
In the 54th minute there appeared no apparent danger when centre-back Sean O'Hanlon and goalkeeper Lee Harper closed in on a through ball - but they paid dearly for a moment's hesitancy and in a moment of panic Harper lashed the ball against the shins of Asamoah - from where it rebounded into an empty net, giving the Shrews striker his 10th goal of the season.
The afternoon steadily became worse for the Dons and, after Stirling was sent off, their defence came under heavy pressure - with the visitors' resistance finally broken when Ashton was on hand to convert a Ben Davies cross.