Castellini was born in Milan. During his club career he initially played for Monza (1965–70), winning the Serie C title, and promotion to Serie B during the 1966–67 season. He later made his Serie A debut with Torino (1970–78) on 30 August 1970, in a 1–0 away win over Ternana; during his time with the club, he won the Coppa Italia in 1971, and the Serie A during the 1975–76 season, collecting 201 Serie A appearances over eight seasons with the club, and 267 club appearances across all competitions.