MK Dons were on the verge of a win which would have taken them out of the bottom four.
But substitute Gary Smith conceded a foul which resulted in Simon Osborn's free-kick being dramatically headed home by Bennett for his first goal of the season.
Earlier MK Dons had been heading for a seemingly well merited away win. Gareth Edds, who scored the goal which kept MK Dons in the division in their final game of the season against Tranmere, was on the mark in a disappointing game in the 73rd minute.
It was only the second goal scored by the Australian this season but it gave MK Dons a vital away point which pleased manager Danny Wilson.
Walsall's young side were unable to raise their game to the frustration of their injured player-manager, Paul Merson.
With four teenagers in the side Walsall were always struggling and were fortunate to escape with Bennett's last-ditch equaliser.
Following back to back home wins MK Dons took their freshly acquired confidence into the game against a young Walsall side with on-loan Steven Quinn making an important contribution.
But neither had a cutting edge to get on top in a goalless first half.
Walsall took a long time to find any attacking ideas, while MK Dons were hardly any better in this department until they went up a gear in the second half.
Their best chance in the first period was created by a cross from Nicky Rizzo but Izale McLeod was slow moving in to make contact.
Matty Fryatt came to life just before the break for the Saddlers with two close-range attempts which were beaten out by goalkeeper Matt Baker.
MK Dons showed more industry with Quinn and Rizzo always in contention.
But it was the speedy McLeod who finally opened up Walsall's defence. His shot was blocked by teenage goalkeeper John Ruddy, but the alert Edds was on the mark to notch the 73rd minute.
Walsall staged their last minute desperate bid for an equaliser with Bennett heading home the vital goal.