Izale McLeod's neat 52nd minute finish had put the Dons on course for their fourth straight home win, but Wrack equalised with his first touch of the ball 13 minutes from the end to keep the Saddlers second.
Walsall started brightly and Martin Butler was unlucky not to earn them a tenth minute penalty when he was dragged down by Sean O'Hanlon in the box, but referee Lee Probert waved play on.
Milton Keynes settled into their stride and Gareth Edds' deflected 30-yard drive was saved unconventionally by Clayton Ince's feet before Clive Platt volleyed the follow up straight into the Walsall goalkeeper's grateful arms from nine yards in the 16th minute.
Walsall's Trevor Benjamin then had a goal incorrectly ruled out for offside in the 36th minute after racing clear and lobbing keeper Lee Harper.
The Saddlers burst out of the blocks after the break and Butler stole in at the near post to head Kevin Harper's teasing cross against the crossbar from six yards four minutes after the re-start.
But, after on-loan Watford debutant Dominic Blizzard and Platt had both gone close for Milton Keynes, McLeod scored his 20th league goal of the season to put the Dons ahead.
The on-rushing McLeod chested down Jon-Paul McGovern's cross in the box and accelerated away from Saddlers defender Chris Westwood before tucking a low finish into the corner of the net from ten yards.
Walsall battled back and Benjamin turned Dons defender Drissa Diallo well in the box, but his angled 12-yard drive was well saved by Harper in the 63rd minute.
And after Dons striker Platt had headed McGovern's pinpoint cross straight at Ince from ten yards, Saddlers substitute Wrack made him pay by equalising at the other end with his first touch of the ball in the 77th minute.
Wrack stole in unmarked at the far post to coolly volley Dean Keates' free-kick low past Harper from seven yards.