Shortly after her marriage, Louise Julie caught the attention of King Louis XV, and was permitted by her husband to become a royal mistress. Although she became the king's mistress in 1732, Madame de Mailly was not officially recognized as his maîtresse en titre until 1738. Madame de Mailly did not use her new position at court to enrich herself or to interfere in politics, unlike her younger sister Marie Anne, who proved to be quite a successful political intriguer.
In 1738, Madame de Mailly received a letter from her younger sister Pauline-Félicité requesting to be invited to court. Madame de Mailly granted her sister's wish, but upon her arrival at court, Pauline-Félicité seduced the king and became his mistress.
While Madame de Mailly remained as the official mistress, the king fell in love with Pauline-Félicité and arranged for her to marry the marquis de Vintimille. Madame de Vintimille quickly became pregnant by the king, and she died giving birth to his illegitimate son, Louis, the duc de Luc, who looked so much like the king that he was called Demi-Louis, "small Louis". Madame de Ventimille's corpse was sent to Lit-the-Parade in the town of Versailles, but during the night, a mob broke in and mutilated the body of "the king's whore".
The king and Madame de Mailly were both devastated by the death of Madame de Vintimille and shocked by the mutilation of her body. In her despair, Madame de Mailly is said to have begun to wash the feet of poor.
Afterwards, the king's best friend, the manipulative duc de Richelieu, began to cast about for another candidate to fulfil his royal friend's desires as he did not want Madame de Mailly to regain the king's affections. He eventually decided upon the younger sister of both Madame de Mailly and Madame de Vintimille, Marie Anne, the widow of the marquis de La Tournelle.
At a masked ball on Shrove Tuesday, 1742, Richelieu led Marie Anne up to the king and introduced them. The beautiful marquise, however, at first rejected the royal advances. As a result, Louis conspired with Richelieu, who was
quite anxious to do anything to bring about a liaison between the king and Madame de La Tournelle because he knew Madame de Mailly did not view him in a kindly light.
But Madame de La Tournelle, who was by far the ablest as well as the most attractive of the de Nesle sisters, unlike Madame de Vintimille and Madame de Lauraguais, was by no means disposed to rest content with a divided empire and secret favours. She insisted that her older sister Madame de Mailly should be dismissed and she herself acknowledged in her place. Louis, who was already wearying of the tears and reproaches of the elder sister, consented; and the countess's post of dame du palais to Queen Marie Leszczyńska was taken away from her, and she was ordered to leave the court. Finding refuge in a convent, Madame de Mailly was to become quite religious.