Sylvester was one of the first 12 followers of St. Francis of Assisi and was the first priest in the Franciscan Order. A descendant of a noble family, Sylvester once sold Francis stones which were to be used to rebuild a church. When, a short while later, he saw Francis and Bernard of Quintavalle distributing Bernard's wealth to the poor, Sylvester complained that he had been poorly paid for the stones and asked for more money.Though Francis obliged, the handful of money he gave Sylvester soon filled him with guilt. He sold all of his goods, began a life of penance and joined Francis and the others. Sylvester became a holy and prayerful man, and a favorite of Francis—a companion on his journeys, the one Francis went to for advice. It was Sylvester and Clare who answered Francis' query with the response that he should serve God by going out to preach rather than by devoting himself to prayer.
Once in a city where civil war was raging, Sylvester was commanded by Francis to drive the devils out. At the city gate Sylvester cried out: "In the name of almighty God and by virtue of the command of his servant Francis, depart from here, all you evil spirits." The devils departed and peace returned to the city.
Sylvester lived 14 more years after the death of Francis and is buried near him in the Basilica of St. Francis in Assisi.