As expected, the Manchester United 'keeper has joined the Championship club on loan until the end of the season after his first team opportunities were limited at Old Trafford.
Johnstone is currently behind David De Gea and Sergio Romero in the pecking order at United and the 23-year-old will hope to gain some valuable first team experience with Steve Bruce's side.
Johnstone told Aston Villa's official website that he was "really excited" to start training with the club and highlighted Steve Bruce as one of the main reasons he chose to join the Birmingham club.
"I'm really pleased to have joined Aston Villa - it's a massive club," he told the club's official website.
"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'm really delighted to be here and I'm happy that it's all sorted. "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".