He was the son of Alexander Seton, Lord Gordon (died 1440) (2nd son of Sir William Seton of that Ilk), by his spouse Elizabeth Gordon (died 16 March 1439), daughter and heiress of Sir Adam Gordon of that Ilk. In 1435 he accompanied the princess Margaret to France to marry the 9th Dauphin of France. In a charter dated 23 February 1439–40, he is styled Sir Alexander Seton of Tullibody, heir of Elizabeth Gordon. The charter confirmed an earlier exchange of lands between Sir William Keith and Margaret Fraser (his maternal grandparents) and William Lindsay, Lord of Byres exchanging lands for that of Dunottar.