Alexander was 16 years old when his father William the Lion died in 1214. The first thing he had to do on taking the throne was to put down a rebellion in Moray. He then invaded England, ostensibly in support of the barons fighting against King John. He invaded again after John's death, but was forced to come to terms with the new English king, Henry III.

He did homage to Henry for his English lands and formally gave up his claim to Northumberland and Cumberland. Then in 1221, he married Henry's sister Joanna, thus further cementing ties between the two countries. The relative peace between the two monarchs was shortlived, but in 1236 Alexander finally renounced his claim to English territory for good.

At home, Alexander had to put down periodic revolts, notably in Argyll, Caithness, and Moray. In 1249 Alexander turned his attention to the Western Isles, still nominally a Norse kingdom. He offered to buy the Western Isles from Hac (Haakon) of Norway, who turned him down. Alexander then raised forces for an invasion, however, he died before the campaign could begin, and he was succeeded by his son, Alexander III.