Isabella was born in 1451 in Ávila, the daughter of John II of Castile and Isabella of Portugal
. At the time of her birth she was second in line for the throne of Castile, behind her elder half-brother Enrique. In 1453 she was displaced by the birth of her brother Alfonso. The following year her father died, and she and Alfonso were placed in the care of Enrique, now reigning as King Henry IV. He moved the children and their mother to Arévalo, where they were kept in relatively poor conditions and had little money. Isabella and her brother were educated mainly by their mother, who instilled in both children an exceptionally strong sense of religious piety. In 1462, just prior to the birth of Enrique's first child, Isabella and Alfonso were summoned to Enrique's court at Segovia. There, they enjoyed a relatively opulent lifestyle, and Isabella continued her education as a member of the Queen's household. At this point she was again moved down the succession by the birth of Juana, who was called la Beltraneja due to the probability of her being the daughter of the Queen's lover rather than the King. In 1464 Juana was promised to Alfonso, who was being propped up by rebels as the true heir to the throne. But in 1468 Alfonso died suddenly and Isabella was adopted by the rebels. She resisted being used as a figurehead, however, and negotiated a settlement with Enrique, who elevated her to first in line for the throne with provisions that she should ask for permission to marry, but could not be forced to marry against her will. A series of betrothals had begun when Isabella was 16, each arranged by Enrique and each one broken for one reason or another. Despite his promises to not force Isabella into marriage, Enrique made several attempts to force her to wed. Isabella herself broke the promise by negotiating a secret marriage promise with Ferdinand of Aragon, her second cousin and the heir to the throne of Aragon. The pair wed in secret in October of 1469. Five years later, Enrique was dead and Isabella ascended to the throne.
Immediately following her ascension to the throne of Castile, Isabella was embroiled in a war with Portugal, who stood behind her niece, Juana. The war lasted until 1479, during which Isabella proved herself a capable ruler and skilled politician, personally putting down a Castilian rebellion and ensuring the throne for herself as well as establishing her daughter, also named Isabella and born in 1470, as the heir presumptive to the throne. Also in 1479 her husband, Ferdinand, inherited the throne of Aragon, becoming Ferdinand II of Aragon. Children John, Joanna, Maria and Catherine were born in 1478, 1479, 1482 and 1485 respectively.
Despite the unstable early start to her reign, Isabella was quick to make many important changes to Castile. In 1474 she took over control of all mints, controlling the production of coinage and the massive inflation that had rendered most coins essentially worthless. In 1476 she established a regular police in the form of La Santa Hermandad, originally vigilante groups that she legitimized and set to the task of enforcing law in Castile, a task that had been largely neglected during the reign of Enrique. Originally operating in Castile, Leon and the Asturias, it was expanded in 1477 to Estremaduras and Andalusia. In 1480 she repossessed a number of royal holdings that had not been paid for sufficiently, vastly improving the state of Castile's finances. That same year she began the reformation of the Royal Council, removing many of the nobility who existed in the Council mainly to exercise personal and political influence and had little in the way of formal duties. She changed much of how government was run, relying on professional administrators rather than nobility. She also developed a weekly court in which she and her husband heard complaints directly from citizens, as well as establishing a comprehensive formal code of ordinances, las Ordenanzas Reales.
In early 1482 she and Ferdinand invaded Granada as part of the exceptionally drawn out Reconquista, which had begun almost 800 years before. The Muslim defenders were poorly organized and internally fractured due to internal disputes, while the Christian forces under Isabella and Ferdinand presented a united and well-organized front. Despite these advantages, their conquest took ten years, during which they retook parts of Granada piece by piece. Finally, at the beginning of 1492, The Granadans officially surrendered and the Reconquista was officially concluded, as all of the Iberian Peninsula was under the control of the Christian monarchies. This success lead to a heightening of racial and religious tension on the Peninsula. That same year, Isabella and her husband sponsored Christopher Columbus
, an Italian
explorer, on his journey to the West, on which he hoped to encounter the Indies. After his successful return in 1493, Spain
entered the Golden Age of Exploration, marking the beginnings of the Spanish Empire. Despite this success and prosperity, all was not well in Isabella's kingdom. In 1480 the Inquisition had been established in Spain. Originally intended merely to regulate the piety of new converts, it became institutionalized, and in 1492 Isabella announced that Jewish subjects had two choices: they could convert to Catholicism, or they could be expelled from Spain. About half of the Jewish population, estimated at about 80,000, fled the country, while the remaining 40,000 chose to convert. Of these conversos
, many came under suspicion of relapsing into their original faith, often resulting in cruel treatment at the hands of the Inquisition. In 1499, an uprising was attempted by Muslim inhabitants of Spain, leading to the breaking of the Treaty of Granada in 1502; all Muslim citizens were ordered to convert to Catholicism or leave the country. In September of 1504 Isabella retired from public affairs. She died on November 26th, 1504 at the age of 53. She was succeed by her daughter, Joanna.