Cyril was born in Alexandria, Egypt, in 376, and was the nephew of Theophilus, the patriarch of the city. When his uncle died in 412, Cyril took his position on the see of Alexandria. He soon began a series of attacks against the Novatians, a religion started by the antipope Novatian. He closed their churches and drove Jews from the city.
In 428, Cyril discovered that the priest/monk Nestorius, the Archbishop of Alexandria, was preaching heretical theology. Cyril sent the heretic a mild expostulation, but to no avail. Both parties then appealed to Pope St. Clementine, and Cyril was appointed to depose Nestorius. In 431, Cyril presided over the Third General Council at Ephesus, attended by some two hundred bishops, which condemned all the tenets of Nestorius and his followers. However, upon the arrival of Archbishop John of Antioch and forty-two followers who believed Nestorius to be innocent, they held a council of their own and deposed Cyril. Emperor Theodosius II had both Cyril and Nestorius arrested but released Cyril on the arrival of papal legates who confirmed the council's actions against Nestorius and declared Cyril innocent of all charges leveled against him.
Two years later, Archbishop John, representing the moderate Antiochene bishops, and Cyril reached an agreement and issued a joint condemnation, and Nestorius was forced into exile.
Cyril died in 444 at Antioch. He was named a Doctor of the Church in 1882.