Cyril was nephew to Theophilus, Patriarch of Alexandria, and succeeded his uncle as patriarch in 412. The first phase of his episcopate was turbulent and at times harsh — he closed the Novatian churches, expelled the Jewish community after intercommunal violence, and was implicated by some sources in the climate that led to the lynching of the philosopher Hypatia in 415 (his direct role is disputed).
His enduring work is theological. When Nestorius — Patriarch of Constantinople from 428 — taught that Mary should be called Christotokos ('Mother of Christ') rather than Theotokos ('Mother of God'), and that Christ existed in two persons united by mutual will, Cyril produced the great series of works — Twelve Anathemas, On the Incarnation, On the Unity of Christ — that articulated the doctrine of the hypostatic union: Christ is one person (hypostasis) in two natures, fully God and fully man.
He presided over the Third Ecumenical Council at Ephesus in 431, which defined Mary as Theotokos and condemned Nestorianism. His Christology, with the later refinements of Chalcedon (451), remains the standard of the Catholic and Orthodox churches. Leo XIII declared him a Doctor in 1882.