He filled the see of Rome at a very important era in the history of the Church, but very little is known of him. The accounts of his papacy preserved in the Liber Pontificalis (7th or 8th century) are little else than a record of the gifts said to have been conferred on the Roman Church by Emperor Constantine I, but it does say that he was the son of a Roman named Rufinus.
During his pontificate were built the great churches founded at Rome by Constantine, e.g. the basilica and baptistery of the Lateran near the former imperial palace where the pope lived, the basilica of the Sessorian palace (Santa Croce in Gerusalemme), St. Peter's Basilica in the Vatican, and several cemeterial churches over the graves of martyrs. All these originally had the entrance at the east end, with the apse to the west.
Saint Sylvester did not himself attend the First Council of Nicaea in 325, but he was represented by two legates, Vitus and Vincentius.
At an early stage copious legend supplemented his scanty history, bringing him into close relationship with the first Christian emperor. These legends were introduced especially into the "Vita beati Sylvestri", which appeared in the East and has been preserved in Greek, Syriac, and Latin in the "Constitutum Sylvestri" – an apocryphal account of an alleged Roman council which belongs to the Symmachian forgeries and appeared between 501 and 508. They also appear in the "Donation of Constantine".
He was said to have baptized Constantine. In fact, according to the contemporary account of Eusebius of Caesarea, the emperor, dying at Nicomedia, was baptized there by another Eusebius, bishop of that city.
The "Donation of Constantine" is a document fabricated in the second half of the eighth century, purporting to be a record by the emperor himself of his conversion, the profession of his new faith, and the privileges he conferred on Pope Sylvester I, his clergy, and their successors. According to it, Pope Sylvester was even offered the imperial crown, which, however, he refused.
Sylvester's legendary relationship to Constantine was important in the Middle Ages. Pope Sylvester II (999-1003), himself a close associate of Emperor Otto III, chose the name Sylvester in imitation of Sylvester I.
In the West, the liturgical feast of Saint Sylvester is on 31 December, the day of his burial in the Catacomb of Priscilla. This is the last day in the year and, accordingly, in German-speaking countries and in some others close to them, New Year's Eve is known as Silvester. In other countries too, the day is usually referred to as Saint Sylvester's Day or the Feast of Saint Sylvester (in French as la Saint-Sylvestre).