Styled Viscount de Lomagne while his father lived, John succeeded him as Count of Armagnac when he died (5 November 1450); soon after, he started a relationship with his sister Isabelle, Lady of the Four-Valleys (Dame des Quatre-Vallées), ten years his junior, whom the chronicler Mathieu d'Escouchy accounted one of the great beauties of France and whose betrothal to Henry VI of England had been under consideration.

But within a few months, John solemnized the union between the two by claiming to have obtained a papal dispensation from Pope Callixtus III, shortly after their third child, a daughter called Rose (or Mascarose), was born.

Authorities were alerted, and a brief was issued for John's arrest, when an investigation revealed that he had forced a forged dispensation out of Antoine d'Alet, Bishop of Cambrai, a magistrate in the court of Rome.

Betraying Louis, Armagnac was part of the league that called themselves Bien public and threatened Paris at the head of 6,000 mounted men.

Pregnant at the time of her husband's death, Joan was transported to the castle of Buzet-sur-Tarn and lived still several years,[4] contrary to Père Anselme's suggestion that she died after being forced to drink a potion (un breuvage) which made her give birth to a stillborn child in ca.