A member of 2009 FIFA U-17 World Cup, Mannini made his competitive debut in 2011–12 Coppa Italia.
In January 2011 he was signed by Internazionale along with national team team-mate Francesco Bardi (from Livorno),[1][2] rejoining defenders Simone Benedetti and Felice Natalino, all of 2009 FIFA U-17 World Cup.
However, he failed to play regularly in the Primavera under-20 team, in which the usual left-back was Cristiano Biraghi.
In June Inter did not exercise the option to sign him in the definitive deal, and in July he left for the newly relegated team Triestina.
He was started the next two games, partnering with Michele Camporese, winning United States in the round of 16.