The 23-year-old could make his debut in Sunday's FA Cup trip to Tottenham after joining on Thursday
He becomes Steve Bruce's first signing for Villa, with the boss having targeted a new goalkeeper in January
Johnstone told the club's official site: "I'm really pleased to have joined Aston Villa - it's a massive club.
"I'm really excited to be here
Aston Villa were interested and it's an opportunity I jumped at
It's one you simply can't turn down.
"I can't wait to work with Steve Bruce too
I've heard very good things about the manager
You can see his track record
"He's good for the club and I can't wait to get going now."
Pierluigi Gollini started the season as the club's first-choice goalkeeper but was replaced by Mark Bunn last month, although the former Norwich goalkeeper has kept just one clean sheet in seven games.
Johnstone is yet to make an appearance for Manchester United but has had loan spells at Scunthorpe, Walsall, Yeovil, Doncaster and Preston.
Manchester United boss Jose Mourinho stated before the transfer window opened that Johnstone will be the only player allowed to leave Old Trafford on loan in January.
Source : PA