The 22-year-old defender signed on loan at stadium:mk until the end of the season, but with his contract at the Checkatrade.com Stadium up in June, has signed a long term deal with Dons for next season and beyond.
He said: “It was sad leaving Crawley in the position they’re in, because I wanted to help them out of it.
“But this was too good an opportunity to turn down.
“This is a team that is fighting at the top of the league and hopefully going to the Championship.”
Walsh will link up with two familiar faces in former Crawley boss Richie Barker and former house mate Kyle McFadzean, and he said that should make things easier to settle in.
“I knew Kyle McFadzean and Richie Barker from their time at Crawley,” said Walsh. “I’ve always kept an eye on Dons to see how they’re doing, and their league position too.
“And everyone knows how well they did in the Capital One Cup against Manchester United - they’re a club on the up.
“Richie took me to Crawley, and I lived with Kyle at Crawley, so it makes it easier to fit in.”
Although his new boss Karl Robinson said Walsh won’t be thrust immediately into the first team, Walsh said he’s comfortable to put in the hard graft off the field, and take his chances when he’s given them to break into the starting 11.
He said: “I’ve just got to bide my time, work hard in training, and be ready when I get my opportunity.”