As the son of Alfonso IX of León and Berengaria of Castile, he witnessed the bad relations between the two kingdoms during his childhood, after the annulment of his parents' marriage.

In 1222, Ferdinand III found himself at odds with Gonzalo Pérez de Lara, Lord of Molina, due to the latter's support of Alfonso IX.

In his last will, executed in 1268, he bequeathed the lordship to infante Fernando de la Cerda, the first-born son of King Alfonso X of Castile.

[4] The marriage between Alfonso of Molina and Mafalda González de Lara took place in 1240, and upon the death of Mafalda's father, Gonzalo Pérez de Lara, infante Alfonso, through his wife, became Lord of Molina in 1243[4] and governed the lordship for the rest of his life, at first jointly with his wife, and then, after her death, alone, just as stipulated in the marriage contract.

However, thanks to a substantial financial compensation, they renounced the throne of León in the Treaty of Benavente, ratified with Ferdinand in the presence of the many magnates and prelates of the realm.

In 1231, while he visited the main cities of León after having taken possession of it, Ferdinand reportedly sent his son Infante Alfonso, then nine years of age and living in Salamanca, to lay waste to the Almohad Caliphate territories around Córdoba and Seville, accompanied by Álvaro Pérez "the Castilian" de Castro and the magnate Gil Manrique.

They exterminated all the inhabitants and seized the town, then proceeded toward Seville and Jerez de la Frontera, and camped there near the Guadalete River.

[7] Emir Ibn Hud, who had gathered a large army of seven divisions, positioned himself between the Castilians and Jerez, forcing them to give battle.

"[7] After his victory in the Battle of Jerez, Álvaro Pérez de Castro the Castilian returned to Castile and handed Infante Alfonso over to his father the king, who was in Palencia.

[10] He attended the Valladolid Cortes of 1258,[11] whose main purpose was to obtain money to fund Alfonso X's designs on the throne of the Holy Roman Empire.

[13] He attended the wedding of Infante Fernando de la Cerda, first-born son and heir of Alfonso X, to Blanche of France, held in Burgos on 30 November 1269.

Later, his remains were transferred to Calatrava la Nueva as specified in his will, and placed in a sumptuous sepulchre which lay under an arch in the main chapel of the monastery's church.