Upon arriving at the royal court, Françoise's attributes and gifts made her alluring to the equally gifted and cultured King, who promptly attempted to seduce her. He began to give favours and gifts to her family. Her husband became a commander of a company. Her elder brother, the Viscount of Lautrec, received the charge of governor of the Milanese duchy. Her two other brothers, Thomas, lord of Lescun, and André, lord of Lesparre or Asparros, were also promoted to high positions in the military by the king. Françoise eventually became the mistress of the king, after a period of resistance, circa 1518.
On 25 April 1519 the Dauphin François was baptised at Amboise. Jean de Châteaubriant and his wife assisted in the ceremony, and Françoise was placed near to the royal princesses, which signified to the Court that she was La mye du roi ("The Sweetheart of the King"). She was the first official mistress that Francis had taken, and he made his affections for her plain to the Court, against her wishes. This greatly displeased his mother, Louise of Savoy, who disliked the de Foix family.
By contrast, Françoise's husband, Jean, though inevitably aware of the affair, showed little interest in the matter: when, in December 1519, Francis sent him to Brittany to negotiate a tax, the Count thanked Francis, and did not raise the matter of the affair. During this time, Françoise remained at the Court, where she was made a lady-in-waiting of Queen Claude, the Duchess of Brittany.
Françoise remained the official mistress of Francis for a decade. She had no political influence, only managing to persuade the King to not disgrace her brother after his defeat at the Battle of Bicocca. However, in 1525, the King was captured at the Battle of Pavia and held captive in Madrid. When he returned to France, the young and blonde Anne de Pisseleu d'Heilly caught his attention. The two women battled for the King's affections for two years before Françoise gave up and returned to Châteaubriant in 1528.