The club beat Scottish rivals Aberdeen and Motherwell to land the youngster after he sought advice from Old Trafford boss Sir Alex Ferguson and decided to move on.
Rankin is set to sign a pre-contract agreement with the Scottish Division One side this week to play for the Dingwall side for the next two seasons.
The 19-year-old will remain officially a Manchester United player until the end of the season but will be allowed to train and take part in friendlies for County for the next six weeks.
The youngster has history with County boss Alex Smith who groomed him for stardom when coaching him at Scotland Under-16,17 and 20 levels.
Smith said on his new signing: "John has great talent but he has reached a point at Old Trafford where it is difficult to progress.
"John didn't want to go to a big club and risk finding himself back in the reserves and that says a lot about his attitude - he is hungry for first-team football.
"I know Motherwell were desperate to sign him but couldn't because of their financial situation.
"Aberdeen were interested too, but he would have been one of many young kids there battling for places."
