Born a Portuguese infanta of the House of Aviz, Isabella was the only surviving daughter of King John I of Portugal and his wife Philippa of Lancaster.

[1] Isabella served as her husband's representative in negotiations with England regarding trade relations in 1439 and those with the rebellious cities of Holland in 1444.

[3] Born in 1397 in Évora, and raised in the Portuguese court in Lisbon, Isabella was the fourth child and only daughter to survive to adulthood.

[13] The marriage contract was drawn up, and Isabella, still in Portugal, was married to Philip the Good by proxy on 24 July 1429, with Roubaix acting as groom.

[14] After an eleven-week journey when the fleet was beset by storms, causing the loss of several ships and much of her bridal trousseau, the convoy reached Sluys on 25 December 1429.

[16][17] Following a week of celebrations in Bruges, the newlyweds then travelled through the main territories of Burgundy: from Ghent (16 January) to Kortrijk (13 February) to Lille, and then to Brussels, Arras, Péronne-en-Mélantois, Mechelen and, by mid-March Noyon, where Isabella, now pregnant, chose to rest through the spring, only leaving when Joan of Arc led a campaign against the nearby Compiègne.

The Portuguese infanta, described by the Burgundian embassy that had negotiated her marriage as appearing to their eyes as a nun when they had first met, and now dressed in loose clothing and flat over-panels to hide her pregnancy, looked particularly dowdy at her new court.

He had showered gifts on her when she had first arrived, and still more when she had become pregnant; yet, he made it clear that he had no intention of keeping his vows of fidelity and chastity.

[22] The child, Antoine, sickly at birth, was christened on 16 January 1431, and soon after both parents left to attend to ducal business.

[22] By the autumn of that year, Isabella was once again pregnant with their second son, Joseph; more importantly, she had spent a long continuous period of time with her husband, and demonstrated her intelligence and abilities, as well as her commitment to Burgundian independence.

At that time she assisted the Flemish harmed by Philip's military actions and, under her protection, advanced Josse van Huerter to her nephew, Ferdinand of Portugal.