Ben Nugent’s dismissal in the 66th minute gave the Dons the chance to impose themselves on the game, and they claimed an away win thanks to second-half strikes from Grigg and Powell.
Yeovil signed Manchester City youngster Jordi Hiwula in the week and he could have given his new side the lead 22 minutes in, heading wide from James Hayter’s cross.
At the other end, Ben Reeves curled a free-kick wide to end a first half starved off chances.
Hiwula was handed an even better chance to bag a debut goal almost straight from the restart, but his panicked strike while staring down the keeper blazed over the crossbar.
Hayter could have put the Glovers ahead seconds later, yet his sharp strike was smartly met by Dons’ stopper Dave Martin.
The tide turned on 64 minutes when Cardiff City loanee Nugent was shown a second yellow for cutting down Grigg in full flow, having received an earlier booking for handball.
It took just four minutes for Karl Robinson’s men to capitalise. First, Grigg raced behind the Yeovil back line to tuck home past on-loan goalkeeper Jake Kean.
And Yeovil’s day got worse six minutes from time when Powell curled home from 18 yards.