Juan II of Castile was the father of Isabel and King of Castile at the time of her birth. He was born on 6 March 1405 in Toro, in the palace of the Royal Monastery of San Ildefonso. He was the son of King Enrique III, known as el Indolente, and Catherine of Lancaster. Juan had two elder sisters: María of Castile, who later married Alfonso V of Aragon, and Catherine of Castile, who married Prince Enrique of Aragon.

His father died in 1406 when Juan was only one year old. A regency was therefore established under his mother and his paternal uncle, Fernando de Antequera, who would later ascend as King Ferdinand I of Aragon.

In 1408, the young page Álvaro de Luna was introduced at court by his uncle Pedro de Luna, then bishop of Toledo and later Pope Benedict XIII. Álvaro quickly gained great influence over Juan II and ultimately became the young king’s most important advisor.

After the death of his mother in 1418, Juan was declared of age in March 1419. By that time, however, he was already entirely under the sway of Álvaro de Luna, with whom his mother had once been involved. Juan II elevated Álvaro to the most powerful position in the kingdom, provoking constant opposition from the noble families and sparking civil wars. Yet despite the turbulence, his court flourished as a center of culture.

From his first marriage, with Maria of Aragon, was born Enrique. Alfonso and Isabel were the children of his second marriage, with Isabel of Portugal.

Juan II died on 20 July 1454 in Valladolid, when Isabel the Catholic was three years old. He was succeeded by his son Enrique.

In his testament it was decreed that, should Isabel’s brothers, Enrique and Alfonso, leave no heirs, Isabel would inherit the throne. The testament also granted her an annual allowance of one million maravedís. Enrique, however, paid no heed to this.