The posthumous daughter of King Edward of Portugal and Eleanor of Aragon, she was born in the Quinta do Monte Olivete Villa, Almada.

On 21 May 1455 in Cordoba,[3] she married as his second wife King Henry IV of Castile who had repudiated his first consort, Blanche II of Navarre, after thirteen years of marriage.

[citation needed] Henry and Joan shared the same maternal grandparents; Ferdinand I of Aragon and Eleanor of Alburquerque (making them first cousins).

She soon fell in love with Bishop Fonseca's nephew; they embarked on a sexual affair, which resulted in Joan bearing her lover two illegitimate sons.

Prior to her banishment, Joan had provoked much criticism in the Castilian court as she allegedly wore dresses that displayed too much décolletage, and her philandering with men was considered scandalous.