Johnson, 24, was quietly released by City in December and has now revealed he has been receiving treatment at the Priory Clinic, which specialises in mental illnesses, for some time.
Johnson, a dynamic midfielder once tipped as a future England international and linked with a big-money move to Liverpool, fell by the wayside after being hit by a succession of injuries.
A loan move to Leicester was cut short a year ago and City reached a settlement over the remainder of a contract which was due to expire this summer.
Johnson last played for City in October 2009 and so never featured under Mancini, who took charge two months later, but the Italian is saddened by his plight.
Mancini said: "Michael Johnson was a guy with a big talent and I am sad for this, sad for him.
"He could do everything with his talent. I am very sorry for him. I hope he can resolve all his problems."