The Italian was relieved of his duties as manager last night following a large amount of speculation.
However, rather than continue in his role with the Citizens, former England international Platt has decided to step down with immediate effect.
"With regret Manchester City announce that David Platt has this afternoon left his role as assistant manager at the club," read a statement on the club's official website.
"David was offered the opportunity to continue his work with us but has declined the invitation. He has decided to leave his role with his close friend Roberto Mancini.
"David has made a significant contribution to the club's success since joining in 2010 and we wish him well with his career wherever that now takes him."
Coach Brian Kidd will take charge of City's remaining two Premier League fixtures.