The facts of Patrick's life are largely obscured by legend. He belonged to a Christian family of Roman citizenship. Captured when barely 16 by Irish marauders and enslaved, he worked for six years as a herder on the slopes of Slemish (near Ballymena, Co. Antrim) or of Croaghpatrick or (most likely) of both. Then, in response to a voice, he escaped and embarked for Gaul.

Patrick spent some years wandering on the Continent and probably visited the Monastery of St. Martin at Marmoutier. He entered the monastery at Lérins and received the tonsure. He returned c.413 to his native Britain and lived for some years with relatives. During this time he had a vision that called him to return to Ireland to Christianize it. Accordingly, he returned to Europe (c.419) to perfect himself as a missionary. The next 12 years were spent in study at Auxerre. In 431, St. Palladius, first missionary bishop sent to Ireland, died; Patrick was consecrated (432) in his place by St. Germanus of Auxerre.

In the winter of 432 Patrick landed near Saul and remained until spring, when he went to Tara and gained his first major converts. He defied the pagan priests of Tara by kindling the Easter fire on Slane, a nearby hill. This challenge to paganism created at first indignation, and subsequently respect, in the court of the high king. Tara became Patrick's headquarters, and with a band of followers he successively converted Meath, Leitrim, Cavan, and W Ireland. Further details of his missions are only generally known.

In 444 or 445, with the approval of Pope St. Leo I, Patrick established his archiepiscopal see at Armagh. St. Patrick's mission was successful; Ireland was almost entirely Christian by the time of his death. He understood and wisely preserved the social structure of the country, converting the people tribe by tribe. Out of his hierarchy, organized by tribal units, developed the Celtic abbot-bishop system. At Patrick's instance, the traditional laws of Ireland were codified. Patrick modified them to harmonize with Christian practice, and he mitigated the harsher ones, particularly those that dealt with slaves and taxation of the poor. He introduced the Roman alphabet. In 457 he retired to Saul, where he died.

He was buried in Downpatrick, which was a great European shrine until its destruction by the English government in 1539. Also enshrined to him is Croaghpatrick. Patrick's connection with Saint Patrick's Purgatory in Lough Derg is undoubtedly only legendary. His personality is said to have been unusually winning, and many legends have become attached to his name. Feast: Mar. 17.