During his spell with "Samp", Mannini won four Coppa Italia and the 1991 national championship, scoring a crucial goal in the latter competition on 19 May in a 3–0 home win against U.S. Lecce. He also appeared in two UEFA finals in the 1990s, winning the UEFA Cup Winners' Cup against R.S.C. Anderlecht and losing the European Cup against FC Barcelona, the latter match played at Wembley.