Young, capped 15 times by his country, underwent a medical yesterday night and the transfer, believed to be worth in the region of £16-20 million (Dh95-120m), was completed Thursday.
He becomes the second pre-season signing at Old Trafford after Sir Alex Ferguson brought England Under 21 international defender Phil Jones from Blackburn Rovers for £16.5 million.
Young, a 25-year-old midfielder, joined Villa from Watford in January 2007 and went on to make 190 appearances in all competitions for the Birmingham club, scoring 38 goals. Watford are due to 15 per cent of anything over the £9.5m Aston Villa paid for him.
"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," recently-appointed 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 the challenge," McLeish also told Villa's official website.