Ibehre fired in from 25 yards out to sink Roberto Di Matteo's side and ensure Peterborough won promotion from League One to the Championship courtesy of their 1-0 victory at Colchester.
Sam Baldock twice missed chances to give the Dons the lead in the first half but he was thwarted by Saddlers goalkeeper Clayton Ince. And the second half looked like being an non-event before Ibehre lit up stadium:mk on the hour mark.
The striker turned Jude Stirling inside out before drilling a thunderbolt to leave goalkeeper Willy Gueret with no chance.
The Dons hit back with Mark Wright seeing his header from 10 yards out saved by Ince before winger Jason Puncheon also tried his luck.
Baldock then saw a low free kick saved by Ince as MK Dons failed to find a way through for an equaliser and they now face the play-off lottery.