Champions capture Young from Villa
The 25-year-old England international underwent a medical yesterday and the transfer was finalised today.
He becomes Sir Alex Ferguson's second signing of the summer after the capture of England U-21 international Phil Jones from Blackburn Rovers.
Villa signed Young from Watford in January 2007 and he went on to score 38 goals in 190 appearances.
However, he made it clear to Villa he would not be signing an extension to his contract, which was due to expire next summer, and the club had little alternative but to sell one of their prize assets.
"Ashley has made a terrific contribution for Villa over the past four and a half seasons and everyone at the club wishes him the best for the future," new Villa manager Alex McLeish said.
"He has become an important player in the England team during that time and I'm sure he will be looking forward to this new challenge."
Young said the lure of Old Trafford proved too much for him to consider remaining at Villa.
"The opportunity to come and play for one of the biggest clubs in the world is one I couldn't turn down," he told MUTV. "It's a chance to hopefully become part of their history by helping to win the 20th title.
"It's a prospect I always looked at from being a child: to play for one of the biggest clubs in the world. I've got that opportunity now so I've just got to take it with two hands."