Baring-Gould’s sources: Aberdeen Breviary, Cologne and German Martyrologies. Not to be confused with Constantine, the successor of king Arthur, nor with Constantine, the Scottish king, who resigned his throne to live as a monk at S. Andrews, in 943. Authority:—The Aberdeen Breviary, John Fordun, John of Tynemouth, and mention in the Life of S. David.

Constantine, son of Padarn, king of Cornwall, was married to the daughter of the king of Brittany, but had the misfortune to lose his wife by death shortly after. He was so deeply attached to her, that he could find no rest in his loneliness. Therefore, resigning his crown, and bidding farewell to his subjects, he crossed over into Ireland, and entered a monastery, without declaring who he was, and whence he came. He was ordered to grind the corn for the brothers; and for seven years he filled this situation. But one day as he sat in the granary, working the rude stone quern with his hands, and thinking himself alone, he laughed, and said, "Is this then, king Constantine of Cornwall, who wore helm and bore shield, who drudges thus at a hand-mill? It is the same, and it is not the same."

Now it happened that one of the brethren was in the granary and heard this, therefore he stole off unperceived to the abbot, and told him who his miller was. Then the abbot called the others, and all the brethren hasted to the mill, and drew Constantine therefrom, and made him one of themselves, instructed him in letters; and finally, by the grace of God, he was ordained priest. And after that, he bade them all farewell, and crossed over into Scotland, and was with S. Columba [Irish abbot of Iona (d. 597), apostle of Scotland] and S. Kentigern, who sent him to preach the Word in Galloway. And afterwards he was made abbot, but of what monastery is not specified, though there can be little doubt it was Glasgow. Now, when he was very old, he went a mission into Kintyre, where he was assailed by the heathen, who knocked him down and cut off his right arm. Having called his brethren about him, and blessed them, he gently bled to death. He is regarded as the first martyr of Scotland.