Born in Monza, Italy, Antonioli developed as a player in the youth ranks of A.C. Milan. He was later loaned out to Monza, making his professional debut with the club in 1986, at the age of 16, in a 1–0 defeat to Juventus in the Coppa Italia. He also made his Milan debut in the Coppa Italia, in a 2–1 home win over Lazio, on 3 September 1988, and made his Serie A debt with the club on 18 April 1992, in a 1–0 win over cross-city rivals Inter. Due to some promising performances, he was brought to the Milan starting lineup at the beginning of the 1992–93 season, featuring in Milan's 2–1 victory over Parma in the 1992 Supercoppa Italiana; however, a string of mistakes, including a dreadful fumble in the Milan derby, which gifted Luigi de Agostini a soft goal, allowed Sebastiano Rossi to reclaim his starting spot in goal. Milan won the league title that season, also reaching the final of the UEFA Champions League. Antonioli made his final appearance for Milan on 29 November 1992, in 1–0 away win over Juventus, in Serie A, making 27 appearances for the club in total.