Villa agree Young transfer to Manchester United
The 25-year-old England international passed a medical in Manchester this week and has agreed a five-year contract.
Villa secured Young from Watford in January 2007 and he went on to make 157 Premier League appearances, scoring 30 goals.
He made 190 appearances in all competitions and scored 38 goals.
"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," said Villa manager Alex McLeish.
"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."
It seemed a hefty outlay when Villa paid Watford a reported initial £8m for the relatively unproven Young during the January 2007 transfer window, but within 12 months it began to look an astute piece of business.
Despite scoring on his debut in a 3-1 defeat at Newcastle, Young was only moderately successfully during his first few months in claret and blue but from then on he made a massive impact.
Having switched from the role of striker to wide midfielder shortly after his arrival, Young was a regular in the side throughout his first full season, missing only one league match (when he was suspended) and scoring nine goals.
Whether he operated as a winger or in a free role just behind the strikers, he was a constant threat to opposition defences and his outstanding form earned him his first England cap in November 2007.
But it was during the 2008-09 campaign that he really blossomed, and apart from regular England selections he collected three prestigious awards.
He was voted Barclays Player of the Month for both September and December (giving him an unprecedented three monthly awards within the calendar year) and was later acclaimed PFA Young Player of the Year.
There was an air of anticipation among Villa fans whenever he was in possession of the ball, and statistics revealed that he led the way for Premier League wide players in terms of goals, assists and crosses.
The highlight of a season in which he missed only two league games was undoubtedly his dramatic stoppage time winner against Everton at Goodison Park, just seconds after the home side had equalised.
It was his second goal of the game and he again finished the season with nine, two of them in cup-ties.
His performances in 2009-10 were terrific and he continued the impressive form he had shown in the previous campaign.
His highlights were scoring a penalty at Anfield in the 3-1 win over Liverpool, producing the centre for Gabby Agbonlahor's winning goal against Manchester United at Old Trafford and bagging the final goal in the incredible 6-4 second leg semi-final win over Blackburn in the Carling Cup.
Last term he operated mainly as a foil for the central striker but later switched back to the wing, impressing in the final weeks as Villa marched up to ninth in the top-flight table.