Alexander was the only son of William the Lion and his wife Ermengarde. 

Born at Haddington, East Lothian on 24 August 1198, he was knighted by King John of England on 4 March 1212. Two years later, he succeeded his father. 

Although Alexander backed the barons who forced John to sign the Magna Carta, in 1221 he married John's eldest daughter Joan in York. She had no children, and died in 1238. 

The following year at Roxburgh, he married Marie, daughter of Enguerand, Baron de Coucy in Picardy - an alliance which raised English fears of a Franco-Scottish alliance.

The long-standing dispute between Scotland and England over the Border was settled in 1237 when, by the Treaty of York, Alexander renounced Scotland's claims to the three northern counties of England in exchange for the honour of Tynedale and the manor of Penrith. 

The border between Scotland and England was now fixed almost exactly on its present line from the lower Tweed in the east to the Solway Firth in the west. 

Alexander died on the island of Kerrera, Argyllshire in 1249 whilst preparing to take the Hebrides from Norway. He was buried at Melrose Abbey, Roxburghshire.