In 1755, she was recruited to replace her sister as a petite maîtresse (unofficial mistress) of the king in Parc-aux-Cerfs by Dominique Guillaume Lebel.
In contrast to her sisters, she was not described as a beauty nor as a prostitute, but as a virtuous girl scarred by pockmarks who supported herself by making false ornamental pearls and as an artist's model.
In 1770, her pension was extended, and it was noted in the report filed in connection to this that she lived a respectable life.
She is noted to have had a good relationship with her sister Marie-Louise O'Murphy, who left Brigitte in charge of her estate and castle Soisy in 1792, when Marie-Louise left for Le Havre during the French Revolution.
Brigitte continued to manage her sister's property when Marie-Louise was imprisoned during the Reign of Terror of Robespierre.