Platt joins Mancini at Man City
Platt, 44, and Italian Mancini were team-mates in Serie A at Sampdoria, then managed by Sven-Goran Eriksson, in the mid- 1990s.
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"David brings a wealth of experience to his role as coach here at City, and he will enhance what is already an excellent team of staff at the club," Mancini said on the City website.
"We had a great relationship as team mates many years ago and I know he will make an important contribution to the club."
Platt had unsuccessful spells as manager of Sampdoria and English club Nottingham Forest before taking charge of the England Under-21 team from 2001 to 2004.
Manchester City, who finished fifth in the Premier League last season and will be competing in the Europa League, are looking like the big summer spenders of English soccer.
Spanish midfielder David Silva is joining from Valencia for an estimated 32 million euros and Germany international defender Jerome Boateng has been signed from Hamburg SV.