 He was an illegitimate son of Alexander Stewart, Earl of Buchan and probably Mairead inghean Eachainn.Alexander held the Earldom of Mar and the Lordship of the Garioch jure uxoris, in right of his first wife Isabel Douglas, Countess of Mar (died 1408).Alexander's marriage to Isabella followed his capture of Kildrummy Castle, and Isabella with it, in 1404.