The 29 year old has signed for £4m and will slot straight into the right back role.
Martin O'Neill has been searching for a naturalised right sided defender since Olof Mellberg left to join Italian giants Juventus.
And Young, who has been capped for England on seven occasions, fits the bill perfectly.
Fans of the Boro will not be happy to see him go as they see him as a consistent performer and the club don't appear to have a replacement.
But they have made a profit, as they signed Young for just £2.5m last summer when, strangely, Villa appeared to turn down the chance to sign him.
However, O'Neill must have realised his mistake and Young should make his debut when Villa play Hafnarfjordur in Iceland next Thursday.