Childhood and time during the reign of Alfonso X El Sabio
Juan Núñez' exact date of birth is unknown. On 2 February 1266, was declared a vassal of Santa María and the Señor de Albarracín. Together with his brother, Nuño González de Lara y León the brothers donated certain goods to Mayor Alfonso, the abbess of the Monasterio de Santa María y San Andrés A few months later on 25 July 1266 together with his brother Nuño González, he was confirmed in a document affirming the commitment of certain nobles of Caleruega to their donations to the King, Alfonso X of Castile. These donations were made for the purpose of acquiring the Monastery at Caleruega and confirming that other ricoshombres with interests in Caleruega accepted this claim over the monastery.
Juan Núñez I de Lara accompanied King Louis IX of France and Theobald II of Navarre during the Eighth Crusade and was with them in Tunis. He participated in this crusade without the consent of Alfonso X, but was nevertheless able to retain all of his lands and titles under his crown.
During the revolts of the nobility of 1272–1273, in which Juan Núñez I supported the positions of his father and other rebel magnates against the infante Philip of Castile. Philip was the brother of Alfonso X of Castile. Juan Núñez tried to remain loyal to the king, who had at the beginning of the rebellion, given him a mission together with Gonzalo Pérez Gudiel, the bishop of Cuenca, to persuade the infante Philip and other magnates to break their agreements with the Kingdom of Navarre. Following this unsuccessful attempt, Juan Núñez accompanied his father, Nuño González de Lara "el Bueno" when he and the rebellious magnates abandoned the Kingdom of Castile and Leon and sought refuge in the Nazarí Emirate of Granada. During his stay in Granada and throughout the actual revolt, he participated together with his father in peace negotiations between the Crown of Castile and the Kingdom of Granada and the nobles in rebellion.
At the beginning of 1273, Juan Núñez I de Lara, who had up until now acted as a mediating force together with the bishop of Cuenca between his father and the King, abandoned the Castilian crown and embraced his role as a rebellious noble. In winter of 1273, at the city of Tudela, the infante Philip of Castile, Lope Díaz III de Haro, Álvar Díaz de Asturias, Nuño González de Lara "el Bueno" and his sons, Juan Núñez I de Lara, and Nuño González de Lara y León amongst other magnates, gathered to pay homage and swear allegiance to Henry I of Navarre. It was here that Juan Núñez I offered the Navarese king a list of grievances that he and the other former Castilian nobles had against their former king, Alfonso X. The former Castillian nobles, in their succession, took new oaths to serve the Kingdom of Navarre as they had previously served the Muslim King of Granada.
Despite the betrayal of many of his nobles, it was Alfonso's wish to pursue his goal of attaining the Fecha del Imperio (title of the Holy Roman Emperor). With this goal in mind, he allowed some of the members of the royal family resume negotiations with the rebel nobles. Amongst these mediators were the infantes Ferdinand de la Cerda and Manuel of Castile, the queen, Violant of Aragon, and Archbishop of Toledo, Sancho de Aragón, son of James I of Aragon. After much negotiation, Alfonso X, advised by his brother Frederick of Castile and Simón Ruiz de los Cameros, the Castilian monarch accepted most of the demands of the exiled nobles, presenting the final agreement to Nuño González de Lara "el Bueno", who in 1273, met with the queen, Violant of Aragon in the city of Córdoba. At the end of that same year, the exiled nobles returned to the Kingdom of Castile. At the same time, Muhammed II al-Faqih, the Sultan of Granada, declared himself and his kingdom, a vassal of Alfonso X, even though the chronicles of King Alfonso X erroneously state that this announcement did not come until 1274. In July 1273, Fernando Rodríguez de Castro, Simón Ruiz de los Cameros, and Diego López V de Haro, the younger brother of Lope Díaz III de Haro were all confirmed as being entered back into the royal diplomas. It was not until early the following year, 1274, when Nuño González de Lara "el Bueno" and his children, amongst other nobles, were allowed back onto the rolls themselves. Juan Núñez' father, Nuño González reappeared on the royal rolls on 24 January 1247 and was reinstated to privileges not received since 14 July 1272. According to the chronicles of Alfonso X, he was reinstated in early 1274 when he was named the Adelantado Mayor de la Frontera de Andalucia.
In 1273, Juan Núñez I de Lara formed part of the embassy sent by Alfonso X to Pope Gregory X. During this trip, the Castilian monarch attempted unsuccessfully to convince the Pope to support his claims to the Imperial Roman throne. Instead, the pope attempted to convince Alfonso X to drop this ambition.
He was further present at the Cortes de Toledo of 1275, where he assisted his father and brother in their goals and where Alfonso X entrusted the government of his kingdom to his firstborn son, the infante Ferdinand de la Cerda. The king announced his decision to travel to France and Germany where he would seek to be crowned officially as the Holy Roman Emperor.
During the absence of Alfonso X of Castile, Juan Núñez I accompanied the infante Ferdinand de la Cerda as he carried out his official duties. On 25 July 1275, the infante Ferdinand de la Cerda died in Ciudad Real whilst awaiting reinforcements in a fight against the Marinid Dynasty in Andalucía. Before his death, Ferdinand asked Juan Núñez I to work in his interests to protect the succession rights of his children, the infantes, Alfonso de la Cerda, and Ferdinand de la Cerda, known as the Infantes de la Cerda, for Ferdinand knew that their succession would be in doubt given his young age and their weak standing at the time of his death. After Ferdinand de la Cerda's death, Juan Núñez I joined the caravan that conducted the infante's cadaver to the Abbey of Santa María la Real de Las Huelgas where he was interred.
After the death of Ferdinand de la Cerda, the infante Sancho, the oldest living son of Alfonso X of Castile and Lope Díaz III de Haro, Lord of Biscay arrived at the Ciudad Real. Lope Díaz III agreed to help Sancho obtain his claim to the throne of Castile and supplant the Cerda infantes in return for protection of his interests. Lope Díaz III gathered his vassals at Ciudad Real to announce his decision to support Sancho's claim to the throne as Alfonso X's legitimate successor. The group also vowed to defend the kingdom against Muslim attackers at Castile's borders.
In early September 1275, Nuño González de Lara "el Bueno", Juan Núñez I's father died at the Battle of Écija where Castilian forces were defeated by forces of the Marinid Dynasty who had invaded Andalucía. His father's head was sent by the Marinid King of Granada to Córdoba in a move he apparently considered to be a gesture of goodwill as Nuño González had previously been in the service of the Emirate of Granada. Nuño González was buried along with the rest of his body at the Convento de San Pablo de Palencia
After the death of his father, Juan Núñez I inherited the title of the Head of the House of Lara and became the preeminent defender of the rights to the throne of the infantes de la Cerda as he had promised Philip of Castile. His principal rival, Lope Díaz III de Haro, Lord of Biscay, became the principal defender of the claims of Sancho of Castile.
Juan Núñez I abandoned the Kingdom of Castile in September 1276 to join the service of Philip III of France. He was joined in his French exile by his brother, Nuño González de Lara y León. Whilst in France, the two brothers became the primary defenders of the Cerda line of succession within the French kingdom. In the Treaty of Vitoria of November 1276, signed by Alfonso X of Castile and Philip III of France, Alfonso X promises to return all lands confiscated from Juan Núñez I de Lara and give them back to him and his heirs although the clauses of this part of the treaty were not vigorously stated.
After Alfonso X and Peter III of Aragon came to an agreement at Vistas de Campillo that the Aragonese monarch would renounce his rights over the Señorío de Albarracín, the infante Sancho IV, who had become an enemy of his father on the issue of the Cerda infantes, solicited the Aragonese monarch to support his claims to the Castilian throne in return for his reinstatement to lordship over Albarracin.
In 1283, Juan Núñez I, together with his son, Álvaro Núñez de Lara, the infante James of Castile, Lord of Cameros, and Juan Alfonso de Haro, continued to wage war against the infante Sancho. The three Cerda supporters along with their vassals had taken power over the municipality of Treviño. Upon hearing this, Sancho sent Lope Diaz III de Haro along with his army to Treviño to confront them. Regardless, no battle took place there between these forces. On 4 April 1284, King Alfonso X of Castile died in the city of Seville. He was succeeded to the throne by his eldest son, Sancho IV of Castile, the enemy of Juan Núñez I and his interests.