Massaro began his career with his local club Monza in Serie B, in 1978, putting on notable performances during his three seasons with the club alongside his more technically gifted teammate, Paolo Monelli, which attracted the attention of larger clubs. In 1981, he was acquired by Serie A club Fiorentina, along with Monelli, making his Serie A debut on 13 September 1981, and his Italy Under-21 debut 10 days later. He instantly became a permanent member of Fiorentina's starting line-up, and he came close to winning the Scudetto during his first season with the club, missing out on the title to Juventus by a single point. He continued to be an important member of the club during his subsequent seasons in Florence.