John was born at Medina del Campo. In his youth he was one of the infantes (princes) of Aragon who took part in the dissensions of Castile during the minority and reign of John II. Till middle life he was also lieutenant-general in Aragon for his brother and predecessor Alfonso V, whose reign was mainly spent in Italy. In his old age he was engaged in incessant conflicts with his Aragonese and Catalan subjects, with Louis XI of France, and in preparing the way for the marriage of his son Ferdinand with Isabella I of Castile which brought about the union of the crowns of Aragon and Castile and which was to create the Kingdom of Spain. His troubles with his subjects were closely connected with tragic dissensions within his own family.  John was first married to Blanche I of Navarre of the house of Évreux. By right of Blanche he became king of Navarre, and on her death in 1441 he was left in possession of the kingdom for his lifetime. But one son, Charles, given the title “Prince of Viana” as heir of Navarre, had been born of the marriage. John quickly came to regard this son with jealousy. After his second marriage, to Juana Enríquez, it grew into absolute hatred, being encouraged by Juana. John tried to deprive his son of his constitutional right to act as lieutenant-general of Aragon during his father’s absence. Charles’s cause was taken up by the Aragonese, however, and the king’s attempt to make his second wife lieutenant-general was set aside.

There followed the long Navarrese Civil War, with alternations of success and defeat, ending only with the death of the prince of Viana, perhaps by poison administered by his stepmother in 1461. The Catalans, who had adopted the cause of Charles and who had grievances of their own, called in a succession of foreign pretenders in a War against John II. His last years John spent contending with these. He was forced to pawn Roussillon, his possession on the north-east of the Pyrenees, to King Louis XI of France, who refused to part with it.  In his old age John was blinded by cataracts, but recovered his eyesight by the operation of couching conducted by his physician Abiathar Crescas, a Jew. The Catalan revolt was pacified in 1472, but John carried on a war, in which he was generally unfortunate, with his neighbor the French king till his death in 1479. He was succeeded by Ferdinand, his son by his second marriage, who was already married to Isabella I of Castile. With his death and son’s accession to the throne of Aragon, the unification of Spain under one royal house began in earnest.