Alexander was the eldest son of Alexander Seton and Christian Cheyne. Alexander married Margaret Murray, and died without issue. Alexander's sister Margaret, who became the heiress of Seton after the deaths of her brothers, married Alan de Wyntoun, and their eldest son William adopted the name and arms of Seton. Alexander's brother Thomas was given as a hostage during the Siege of Berwick in 1333 and was executed, due to the terms of the hostages being broken. During the same siege, another brother, William, drowned during an attack on the English fleet. Alexander's father, was the Governor of Berwick who surrendered Berwick after the Scots were defeated at the Battle of Halidon Hill in 1333.