Mancini paid the price for a poor season which saw the club beaten by Wigan in Saturday's FA Cup final at Wembley after surrendering the Premier League title to arch-rivals Manchester United.
Platt was offered the chance to stay on at the club which he joined in 2010 but decided to join Mancini in leaving.
"With regret Manchester City announce that David Platt has this afternoon left his role as assistant manager at the club," read a statement on City's 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."
Platt, 46, and Italian Mancini were team-mates in Serie A at Sampdoria - then managed by Sven-Goran Eriksson - in the 1990s after the England midfielder had spells at Bari and Juventus.
Platt, who also played for Aston Villa and Arsenal, returned to Sampdoria as manager before taking charge of Nottingham Forest and then the England Under-21s from 2001 to 2004.