He was noted for killing Thomas of Lancaster, Duke of Clarence, heir to the English throne, at the Battle of Baugé.
Buchanan joined the Scottish army which was sent to France to aid the French against the English invasion of Henry V. After the Treaty of Troyes, resistance was led by the Dauphin Charles.
During the battle, Sir John Carmichael of Douglasdale broke his lance, unhorsing the Duke of Clarence.
[2] Sir Alexander subsequently held the prince's coronet aloft on his lance, bringing such dismay to the English that the battle was lost to them.
In recognition of this, the Clan Buchanan coat of arms to this day depicts a ducal coronet being held up in triumph.