It is the 23-year-old’s eighth loan move away from Old Trafford. He has previously been at Scunthorpe United, Walsall, Yeovil, Doncaster Rovers (twice) and his hometown club Preston (twice).
Johnstone was part of Preston’s promotion-winning team to the Championship in 2014-15 and Steve Bruce was really keen on signing the keeper.
He is the first signing made by Villa boss Steve Bruce since the former United favourite took charge in October.
Villa registered their interest in Johnstone last month and the move was agreed over the last week. An announcement was imminent after Johnstone was seen enjoying a warm embrace with United No.2 Sergio Romero following the warm-up at West Ham.
Johnstone, who is yet to make a senior appearance for United, signed a new contract until 2018 three months ago and outlined his intention to move on loan in January later that day. However, Johnstone is looking to make a series of appearances for Villa.
Villa are 12th in the Championship but just seven points off the last Play-off place under Bruce, who clinched four promotions to the Premier League with Birmingham and Hull.