 Alexander sat on the jury of 21 knights and peers that convicted his first cousin Murdoch Stewart, Duke of Albany, and two of his sons of treason in 1424, leading to their execution and the virtual annihilation of the Stewarts of Albany.Alexander first married Isabel Douglas, Countess of Mar (died 1408).Alexander later married Marie van Hoorn, daughter of Willem, Lord of Duffel, in 1410.