Alexander was born at Roxburgh, the only son of Alexander II by his second wife Marie de Coucy. Alexander III was also the grandson of William the Lion. Alexander's father died on 8 July 1249 and he became king at the age of seven, inaugurated at Scone on 13 July 1249.
The years of his minority featured an embittered struggle for the control of affairs between two rival parties, the one led by Walter Comyn, Earl of Menteith, the other by Alan Durward, Justiciar of Scotia. The former dominated the early years of Alexander's reign. At the marriage of Alexander to Margaret of England in 1251, Henry III of England seized the opportunity to demand from his son-in-law homage for the Scottish kingdom, but Alexander did not comply. In 1255 an interview between the English and Scottish kings at Kelso led to Menteith and his party losing to Durward's party. But though disgraced, they still retained great influence, and two years later, seizing the person of the king, they compelled their rivals to consent to the erection of a regency representative of both parties.
On attaining his majority at the age of 21 in 1262, Alexander declared his intention of resuming the projects on the Western Isles which the death of his father thirteen years before had cut short. He laid a formal claim before the Norwegian king Haakon. Haakon rejected the claim, and in the following year responded with a formidable invasion. Sailing around the west coast of Scotland he halted off the Isle of Arran, and negotiations commenced. Alexander artfully prolonged the talks until the autumn storms should begin. At length Haakon, weary of delay, attacked, only to encounter a terrific storm which greatly damaged his ships. The Battle of Largs (October 1263) proved indecisive, but even so, Haakon's position was hopeless. Baffled, he turned homewards, but died in Orkney on 15 December 1263. The Isles now lay at Alexander's feet, and in 1266 Haakon's successor concluded the Treaty of Perth by which he ceded the Isle of Man and the Western Isles to Scotland in return for a monetary payment. Norway retained Orkney and Shetland until 1469 when they became a dowry for James III's bride Margaret of Norway.