Soon after assuming the throne, he conducted a campaign to reincorporate the Balearic Islands into the Kingdom of Aragon - which had been lost due to the division of the kingdom by his grandfather, James I of Aragon. Thus in 1285 he declared war on his uncle, James II of Majorca, and conquered both Majorca (1285) and Ibiza (1286), effectively reassuming suzerainty over the Kingdom of Majorca. He followed this with the conquest of Minorca - until then, an autonomous Muslim state (Manûrqa) within the Kingdom of Majorca - on 17 January 1287, the anniversary of which now serves as Minorca's national holiday.
He initially sought to maintain Aragonese control over Sicily early in his reign by supporting the claims to the island of his brother, James II of Aragon. However, he later pressed his brother to retract the claims and instead supported the claim from the Papal States.
His reign was marred by a constitutional struggle with the Aragonese nobles, which eventually culminated in the articles of the Union of Aragon - the so-called "Magna Carta of Aragon", which devolved several key royal powers into the hands of lesser nobles. His inability to resist the demands of his nobles was to leave a heritage of disunity in Aragon and further dissent amongst the nobility, who increasingly saw little reason to respect the throne, and brought the Kingdom of Aragon close to anarchy.
During his lifetime a dynastic marriage with Eleanor, daughter of King Edward I of England, was arranged. However Alfonso died before meeting his bride. He died at the age of 26 in 1291, and was buried in the Franciscan convent in Barcelona; since 1852 his remains have been buried in Barcelona Cathedral.