Alexander was a son of Alexander de Seton (d before 1296), of the Seton family based in the Lothians, and would play an important role in the Battle of Bannockburn. His defection to the Scottish side on the eve of the battle has become part of the folklore that has grown up around this battle but in fact was a reflection of his changeable allegiances to whichever side best served his interests.
After being captured in the pursuit after the Battle of Methven in 1306 by John Mowbray, he subsequently did homage to Edward I of England to swear his loyalty. However two years later he and two other knights Sir Gilbert Hay and Sir Neil Campbell met at Cambuskenneth Abbey on 9th September 1308 to swear on the Eucharist “to defend the liberties of the realm and of Robert the recently crowned king against all men, English, Scots or French.” This oath did not last for long though, and Alexander served between 1308 and 1312 on the jury set up by Edward II of England to forfeit the Lothian lords that had defected to Robert the Bruce.
He defected to Robert the Bruce’s camp on the eve of Bannockburn with the alleged words “Sir, this is the time if ever you intend to undertake to reconquer Scotland. The English have lost heart and are discouraged.” It gave Bruce and his troops the encouragement they needed to launch an assault on the English.
After Bannockburn, Alexander’s future lay with Robert the Bruce and by January 1328 he was the Governor of Berwick, a key strategic position. During the defence of this position in 1333 he saw his son Thomas executed before him. Indeed all four of his sons by his wife Christian, daughter of Cheyne of Straloch, predeceased him and he was succeeded on his death about 1349 by his granddaughter Margaret who had married Alan de Wyntoun.