 His paternal grandmother was the daughter and co-heiress of Donnchadh, Earl of Lennox.Through his mother he was also a descendant of Thomas Randolph, 1st Earl of Moray, nephew of King Robert I of Scotland.Through his son Matthew Stewart, 2nd Earl of Lennox, Stewart was the great-great-great-grandfather of Henry Stuart, Lord Darnley, husband of his first cousin Mary, Queen of Scots and father of James VI, King of Scotland, who became James I, King of England.