Lord Boyd conspired with his brother, Sir Alexander Boyd, and obtained possession of young James III in 1466. He was then made by Act of Parliament sole Governor of the Realm, Lord Chamberlain of Scotland for life, and Lord Justice General in 1467. Early in that year, he procured the marriage of his eldest son, Thomas, (created Earl of Arran for that occasion) with Mary Stewart, Princess of Scotland, elder sister of James III, which aroused the jealousy of the other nobles and made his eventual downfall inevitable, since King James III regarded the marriage of his sister as an unforgivable insult.