Janko is United's second signing of the week after the Red Devils completed the signature of Marouane Fellaini late on transfer deadline day.
The 17-year-old is said to have impressed representatives of the Old Trafford club at the FIFA Blue Stars Tournament earlier this year.
The youngster, who has represented Switzerland at youth level, was signed following a successful trial spell at the Aon Training Complex.
Janko was born in Zurich to a Gambian father and a Swiss/Italian mother.
He is expected to link up with the United youth side initially.