Premier League - Young signs for United
A club statement read: "Manchester United is delighted to announce it has reached agreement with Aston Villa for the transfer of Ashley Young.
"The player passed a medical in Manchester this week and has agreed a five-year contract."
There was no mention of a transfer fee. It has been reported that Villa will receive around £15 million for the England international.
Young said in an interview with MUTV that he will miss Villa, but "the opportunity to come and play for one of the biggest clubs in the world is one I couldn't turn down".
"It's a chance to... hopefully become part of the history by helping to win the 20th title," he said.
"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."
England defender Rio Ferdinand, who helped United seal a record 19th English title this season, described Young as "another top, hungry and young player" on Twitter.
The 25-year-old Young, who can play as a left winger or second striker, will give manager Alex Ferguson more options in attack as the club seek to build on last season's title and Champions League final appearance.
The speedy Young, who joined Villa in early 2007 from Watford, scored seven goals for the Midlands club last season and provided the highest number of assists for them in their league campaign with 11.
Having made his England debut in November 2007, he did not make Fabio Capello's squad for the 2010 World Cup in South Africa but has been back in the frame for his country's Euro 2012 qualifying campaign.
In the most recent of his 15 England caps, he came off the bench to change the game against Switzerland earlier this month, scoring the equaliser in a 2-2 draw and injecting some energy into a struggling team.