Dionysius (Greek Dionysios) was born to a pagan philosophical Alexandrian family around 190 AD and converted to Christianity as a young adult. He studied under Origen at the Catechetical School of Alexandria, succeeded the brief intermediary head Heraclas as head of the school around 232, and became Patriarch (Bishop) of Alexandria around 247.
His seventeen-year episcopate (247–264) was the longest and most consequential 3rd-century Alexandrian patriarchate. He governed through the Decian persecution of 249–251 (during which he himself fled into the Libyan desert to preserve the leadership of the Alexandrian church), the great plague of 250–270 (which killed perhaps a third of the Roman empire's population — Dionysius's pastoral letters during the plague describing the Christians' care for the sick, including pagans, became one of the foundational early Catholic texts on care of the sick during epidemics), and the Valerian persecution of 257–260 (during which he was again forced into exile to Kefro and Colluthion in Libya, from which he continued to govern the Alexandrian church by letter).
His theological output is foundational: his Refutation of the Anti-Origenists defended Origen's broad theological method against early Origenist controversies, his letters on the lapsi (the Christians who had performed pagan sacrifice during persecution and now wished to return to the Church) established the principles of post-persecution reconciliation, and his careful philological-historical analysis of the Apocalypse of John (preserved in fragments by Eusebius) was the first serious Christian biblical-critical examination of the authorship of a canonical New Testament book — Dionysius concluded that the Apocalypse and the Gospel of John were not by the same author, a methodological insight that anticipated by 1600 years the modern historical-critical study of the Johannine corpus.