The 20-year-old is still waiting to make his first appearance for the Premier League champions, but will hope to change that after agreeing a deal until June 2017.
Preston-born Johnstone has spent time on loan at Oldham Athletic, Scunthorpe United, Walsall and most recently Yeovil Town, where a proposed three-month spell was curtailed after just one game in August as he sustained a finger injury.
Capped at under-16, under-17, under-19 and under-20 level with England, Johnstone will be determined to catch the eye of United manager David Moyes.
Get the best features, fun and footballing frolics straight to your inbox every week.
Thank you for signing up to Four Four Two. You will receive a verification email shortly.
There was a problem. Please refresh the page and try again.