The 24-year-old has also revealed he has been receiving treatment at the Priory Clinic, which specialises in mental illnesses, for "a number of years".
Johnson, once tipped as a future England international, was released by City in December after struggling for fitness.
He had not played for the club since October 2009 after being blighted by a succession of injuries, including a ruptured cruciate ligament.
A loan spell at Leicester last season was cut short and there were rumours that he lacked the dedication of a top-flight footballer.
City did not publicise Johnson's release at the time but news of it emerged this week and it now seems his career could be over.
Johnson told the Manchester Evening News: "I am more disappointed than anyone but that's the way it goes.
"I have been attending the Priory clinic for a number of years now with regard to my mental health and would be grateful if I could now be left alone to live the rest of my life."