Juan II succeeded to the throne when he was only a year old. His regency was well administered under his mother Catherine and his uncle Ferdinand I of Aragón until 1419.

The king is remembered as a great patron of literature, who sponsored the birth of Castilian lyric poetry, until that time missing from the culture. He is also remembered for his choice of Alvaro de Luna to take over the tiresome business of running the country.

Luna has long been recognized as one of the best administrators Spain ever had, and because of his dramatic fall from favor and public execution he became a well-known figure in both poetry and drama.

The story of Alvaro de Luna was a covert way for later authors, such as Tirso de Molina, to deal with the topic of homosexuality. The love between Juan and Alvaro, for which there are many sources, is worthy of a novel as well.

The relationship began when the king was 3, with the appointment of Alvaro as his page (doncel). The bond which quickly emerged between them was so strong that those hostile said the king was victim of an hechizo or enchantment; this in fact became a euphemism in Spain for "inappropriate" sexual desire.

When the young king was 7, his mother exiled Alvaro and kept the king virtually a prisoner, a period that ended only with her death seven years later, when the king was 14 and Alvaro 29. After her death, the lovers were reunited and stayed together for 35 years, sharing the bedroom.

Juan and Alvaro were immediately reunited, and Alvaro, a brilliant conversationalist, was the favorite of many court ladies. He is also the author of one of the earliest and most balanced Spanish defenses of women against misogynist charges.

Save for a later period when the king was again prisoner and Alvaro exiled, which was intended to end their relationship, Juan and Alvaro remained together for thirty-five eventful years. They struggled together against a hostile aristocracy, sometimes fleeing together from superior force.

The end came with Juan's remarriage after his first wife's death. His chief counsellor and lover was Don Alvaro de Luna, an accomplished administrator and diplomat, arranged an advantageous second marriage to a Portuguese princess in 1450 when John II's first wife died.

Juan II's new wife, mother of the prudish Isabella the Catholic, resented de Luna's influence (and was perhaps also jalous...). She was able to force the dismissal of Alvaro, then she had her husband execute de Luna in 1453. Juan II is reputed to have died, a year later, of remorse for this treacherous act.