John was the son of the Portuguese King Peter I and his lover Inês de Castro, a Galician noblewoman who had arrived in Portugal as a lady-in-waiting to Infanta Constance of Castile, recently married to Pedro I (royal heir at the time).

The Duke Infante lived at the Court of Ferdinand I, and was esteemed as a good knight, a dexterous rider and able hunter, valiant in shooting.

Leonor persuaded her brother João Afonso Telo 6th Count of Barcelos and Alcaide-Mór (noble magistrate) of Lisbon, to insinuate the idea in the mind of the Infante that his marriage to her daughter, the heiress presumptive Beatrice (who was promised to John I of Castile), would find favour with the people and clear his way to the throne.

In one single coup, Leonor Teles got disentangled of her sister and brought such opprobrium upon the name of her brother-in-law, that his hopes of succeeding to the throne were much diminished.

Infante John, realizing he had been ensnared by the machinations of Leonora, retired to the Douro and then to Castile, avoiding the persecution that was encouraged by the cunning Leonor.

When the Castilian army invaded Portugal, John joined its ranks and laid siege to Elvas, which held firm.

Meanwhile, his connections with Castile eventually cost him the throne, thanks to the argumentation of Master João das Regras at the Cortes of Coimbra in 1385.

[5] Castile's truces with England and Portugal from 1388 onwards seem to have eclipsed Duke John from any significant political role, and he kept a relatively low profile until his death.

The Portuguese genealogist Anselmo Braamcamp Freire cites a letter from John I of Portugal, dated 20 September 1400, in which he refers to his deceased namesake half-brother,[11] but he also suggests that the date might be wrong due to statements that Duke John was active in 1402 in confirming privileges given by Henry III of Castile to Palencia Cathedral.