Scottish Protestant noble, an adherent of John Knox and a sometime supporter of Mary, Queen of Scots.

He was a more pronounced reformer than his father, the 4th earl, whose English sympathies he shared, and was among the intimate friends of John Knox. In March 1557 he signed the letter asking Knox to return to Scotland; in the following December he subscribed the first “band” of the Scottish reformers; and he anticipated Lord James Stewart, afterward the regent Moray, in taking up arms against the regent, Mary of Guise, in 1558. Then, joined by Stewart and the lords of the congregation, he fought against the regent and took part in the attendant negotiations with Elizabeth I of England, whom he visited in London in December 1560.

When in August 1561 Mary, Queen of Scots, returned to Scotland, Glencairn was made a member of her council; he remained loyal to her after she had been deserted by Moray, but in a few weeks rejoined Moray and the other Protestant lords, returning to Mary's side in 1566. After the Queen had married the Earl of Bothwell she was again forsaken by Glencairn, who fought against her at Carberry Hill and at Langside. The Earl was always to the fore in destroying churches, abbeys, and other “monuments of idolatry,” and his short satirical poem against the Grey Friars is printed by Knox in his History of the Reformation.