Although he started working at a young age, Mazzola continued to cultivate his passion for football; he was the leader of Tresoldi, the team of Cassano d'Adda, who observed him playing in the fields and introduced him to their youth team in 1934. He briefly moved to Fara d'Adda and then returned to Tresoldi. He played as a centre midfielder and right-sided midfielder, even though he was already a multi-purpose player. He participated in his first season with Tresoldi in 1935–36, and the following year he played for the first team, earning 10 lire per game.