The 24-year-old, who is yet to make a Premier League appearance for United, has previously had loan spells at clubs including Scunthorpe United, Preston North End and Doncaster Rovers.
“I am delighted to be back,” Johnstone told Villa’s website. “I really enjoyed my last spell here – it’s a great club with top players and passionate fans.
“I can’t wait to get started now. I believe it can be a special season for all of us and I will be working hard each and every day to produce the kind of performances that will help us achieve what we’re aiming for.”
United are currently in Los Angeles for pre-season, with David de Gea and Sergio Romero set to share goalkeeping duties yet again for Jose Mourinho’s side.