The 24-year-old broke on to the Premier League scene in 2006 and quickly became tipped for an England call-up following a string of impressive displays.
However, after injury problems and weight issues the Manchester-born midfielder's career now appears to be over after City decided to pay out the remainder of his contract, which was due to expire this summer.
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"I am more disappointed than anyone but that’s the way it goes," Johnson told The Manchester Evening News.
"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."
Johnson had often been compared to Manchester City legend Colin Bell during his spell at the club, and the former England international admits he is disappointed with the way the 24-year-old's career has panned out.
"It’s a terrible shame," he added. "It’s hard to accept when your career ends early and for his to end at his age will be really difficult to take."
Johnson made 27 appearances for Manchester City and scored two goals, making his last appearance as a substitute against Scunthorpe in the League Cup in 2009.
He also made seven appearances during an unsuccessful loan spell at Leicester City last season.