Through his mother, daughter of Álvaro of Braganza (4th male son of Fernando I, 2nd Duke of Braganza), John of Lencastre was close related to the most prestigious aristocratic families of the Kingdom (namely the Duke of Braganza, the Marquis of Ferreira, the Marquis of Vila Real, the Count of Vimioso, the Count of Portalegre).
When he was 12 years old, he served crown prince John (who became later King John III of Portugal) and King Manuel I granted him the title of Marquis of Torres Novas by a royal decree issued on March 27th, 1520.
In that time, the royal house announced the mariage of Infante Ferdinand, Duke of Guarda (King Manuel I younger son) to the richest and prestigious heir Dona Guiomar Coutinho, 5th Countess of Marialva and the 3rd Countess of Loulé. However, John of Lencastre declared he had secretly married the richest bride. The scandal in the Court ended when King John III ordered the Marquis imprisonment in the Castle of São Jorge, in Lisbon, allowing the celebration of the Infante Ferdinand's marriage.
After nine years, when he was released, he went to live in Setúbal, maybe ashamed for the situation, and he only returned to the Court in 1535, to escort Infante Luis, Duke of Beja, who was leading the Portuguese fleet in Charles V’s crusade against Tunis. In that year King John III granted him the title of Duke of Aveiro.
After his return from Madrid, in 1537 (where he was sent by King John III to present formal condolences to Charles V, when his wife, Empress Isabel of Portugal, died) John of Lencastre, 1st Duke of Aveiro, decided to marry to Dona Juliana de Lara, daughter of the 3rd Marquis of Vila Real.
John of Lencastre and Juliana de Lara had two sons: