Young, who agreed a five-year contract in a deal estimated to be worth around 15 million pounds, is manager Sir Alex Ferguson's second June buy.
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The United boss is bidding to strengthen his ranks following last season's 19th Premier League title and Champions League final appearance in which they were largely outclassed by a rampant Barcelona side.
The 25-year-old winger, who can also play as a second striker, joins England under-21 defender Phil Jones, who moved to United from Blackburn Rovers in another big-money move earlier this month.
"Manchester United is delighted to announce it has reached agreement with Aston Villa for the transfer of Ashley Young," the club said in a statement on their website.
"The player passed a medical in Manchester this week and has agreed a five-year contract."
Young joined Villa from Watford in 2007 and has since built a reputation as a consistent Premier League performer with an impressive change of pace and an eye for a pass.
His international career has been stop-start, however, and he has collected only 15 caps since making his debut in 2007.
He did not even make Fabio Capello's England squad for the 2010 World Cup in South Africa.
But he forced his way back into the international setup and showcased his ability to change a game when he came off the bench to score the equalizer in England's 2-2 draw with Switzerland earlier this month.
He was Villa's creator-in-chief last season, notching his team's highest number of assists as he helped them to a ninth-place Premier League finish following a poor start to the campaign.
"The opportunity to come and play for one of the biggest clubs in the world is one I couldn't turn down," Young told the club website.
"It's a prospect I always looked at from being a child."
Young is unlikely to be United's final summer capture, with Atletico Madrid goalkeeper David de Gea expected to arrive at Old Trafford following Spain's European Under-21 Championship campaign.
