According to Sebastian's fifth-century Acta Sanctorum, still attributed to Ambrose by the seventeenth-century hagiographer Jean Bolland, and the briefer account in Legenda Aurea, he was a man of Gallia Narbonensis who was taught in Milan and appointed as a captain of the Praetorian Guard under Diocletian and Maximian, who were unaware that he was a Christian.
Sebastian was known for having encouraged in their faith two Christian prisoners due for martyrdom, Mark and Marcellian, who were bewailed and entreated by their family to forswear Christ and offer token sacrifice. His aura was said to have cured a woman of her muteness, and that the miracle instantly converted 78 persons.
According to tradition, Mark and Marcellian were twin brothers and were deacons. They were from a distinguished family and were both married, living in Rome with their wives and children. The brothers refused to sacrifice to the Roman gods and were arrested. They were visited by their father and mother, Tranquillinus and Martia, in prison, who attempted to persuade them to renounce Christianity.
Sebastian ended up converting Tranquillinus and Martia, as well as Saint Tiburtius, the son of Chromatius, the local prefect. Nicostratus, another official, and his wife Zoe were also converted. According to the legend, Zoe had been a mute for 6 years. However, she made known to Sebastian her desire to be converted to Christianity. As soon as she had, her speech returned to her. Nicostratus then brought the rest of the prisoners; these 16 persons were also converted by Sebastian.
Chromatius and Tiburtius became converts; Chromatius set all of his prisoners free from jail, resigned his position, and retired to the country in Campania. Mark and Marcellian, after being concealed by a Christian named Castulus, were later martyred, as were Nicostratus, Zoe, and Tiburtius.
Diocletian reproached Sebastian for his supposed betrayal, and "he commanded him to be led to the field and there to be bounden to a stake for to be shot at. And the archers shot at him till he was as full of arrows as an urchin (hedgehog)," leaving him there for dead. Miraculously, the arrows did not kill him. The widow of Castulus, Irene of Rome, went to retrieve his body to bury it, and found he was still alive. She brought him back to her house and nursed him back to health. The other residents of the house doubted he was a Christian. One of those was a girl who was blind. Sebastian asked her "Do you wish to be with God?", and made the sign of the Cross on her head. "Yes", she replied, and immediately regained her sight. Sebastian then stood on a step and harangued Diocletian as he passed by; the emperor had him beaten to death and his body thrown in a privy. But in an apparition Sebastian told a Christian widow where they might find his body undefiled and bury it "at the catacombs by the apostles."
Of the miraculous effect of the example of Sebastian, the Golden Legend reports,
"And Saint Gregory telleth in the first book of his Dialogues that a woman of Tuscany which was new wedded was prayed for to go with other women to the dedication of the church of Sebastian, and the night tofore she was so moved in her flesh that she might not abstain from her husband, and on the morn, she having greater shame of men than of God, went thither, and anon as she was entered into the oratory where the relics of Saint Sebastian were, the fiend took her and tormented her before all the people."
Sebastian was also said to be a defense against the plague. The Golden Legend transmits the episode of a great plague that afflicted the Lombards in the time of King Gumburt, which was stopped by the erection of an altar to Sebastian in the Church of Saint Peter in the Province of Pavia.
Because Sebastian had been thought to have been killed by the arrows, and yet was not, and then later was killed by the same emperor who ordered him shot, he is sometimes known as the saint who was martyred twice.