bishop generally credited as the first Christian missionary to Scotland, responsible for widespread conversions among the Celts (Celtic Church).

The two primary historical sources about Ninian's life and work are of dubious reliability. According to one, a 12th-century life by abbot Aelred (Ethelred) of Rievaulx (Aelred of Rievaulx, Saint), Ninian was the son of a Christian Briton chieftain. He made a pilgrimage to Rome, where he was consecrated as a bishop, and, in Aelred's narrative, traveled through Gaul on his return journey, along the way befriending St. Martin of Tours. An earlier source, the Venerable Bede's 8th-century Ecclesiastical History of the English People, implies that Ninian began the conversion of the Picts (Pict), a notion based on even earlier—and not entirely trustworthy—accounts of the period.

More certainly, Ninian was the first bishop of Galloway. That he established his see at Whithorn, Caledonia, is a supposition borne out by modern anthropology. There, in about 397, he built a whitewashed stone church (hence Whithorn, or White House, from the Anglo-Saxon Huitaern; Latin Candida Casa)—a notable departure from the customary wooden churches of the Britons. The monastery that he established at Whithorn was, by the 6th century, a leading Anglo-Saxon monastic centre.

Historically, there is little doubt that Ninian carried out his mission in Scotland, although there is some confusion about the areas that he visited. Modern scholars believe that, though his influence among the Picts may have been overestimated, his success with the Celts was evidently much greater. Indisputable evidence of his influence survived in the large number of churches dedicated to him throughout Scotland and in several locations in northern England, and it is generally agreed that his missionary work prepared the foundation for the later efforts of Saints Columba and Kentigern.

St. Ninian's shrine at Whithorn drew many pilgrims, among them King James IV of Scotland, who was a regular visitor. The Roman Catholic diocese of Galloway retains Candida Casa as its official name.