Kevin Davies tried to double the advantage soon after, however the 36-year-old's powerful strike was parried by goalkeeper David Martin.
The former Bolton striker tried his luck once more but nodded Humphrey's cross inches wide of the left-hand post.
Instead Dons' Jason Banton levelled the score in the 31st minute with a stunning long-range strike that flew past Declan Rudd and into the top corner.
The hosts began the second half in similar high-tempo fashion and Garner flicked the ball on to Davies, who struck his shot wide.
But Karl Robinson's side responded with another equaliser with 27 minutes remaining after Humphrey conceded a penalty and Williams slotted the ball into the bottom corner.