Through the influence of a distant kinswoman, Mme de Choisy, Louise was named Maid of honour to Princess Henrietta Anne of England, sister of King Charles II of England, who was about her own age and had just married Philippe I, Duke of Orléans, the King's brother. Henrietta (known as Madame) was extremely attractive and joined the court at Fontainebleau in 1661. Her friendly relationship with King Louis XIV, her brother-in-law, caused some scandal and fed rumors of a romantic affair.
To counter these rumors, the King and Madame decided that Louis should pay court elsewhere as a front, and Madame selected three young ladies to "set in his path", Louise among them. The Abbé de Choise reported that the seventeen-year-old girl "had an exquisite complexion, blonde hair, blue eyes, a sweet smile . . . [and] an expression [at] once tender and modest." One of her legs was shorter than the other, so Louise wore specially made heels.
Louise had been at Fontainebleau only two months before becoming the king's mistress. Although she was intended to divert attention from the dangerous flirtation between Louis and his sister-in-law, Louise and Louis soon fell in love. It was Louise's first serious attachment and she was reportedly an innocent, religious-minded girl who initially brought neither coquetry nor self-interest to their secret relationship. She was not extravagant and was not interested in money or titles that could come from her situation; she wanted only the King's love.
In February 1662, the couple fell into conflict. Despite being directly questioned by the King, Louise refused to tell her lover about the affair between Henrietta and the comte de Guiche. Coinciding with this, Jacques-Benigne Bossuet delivered a series of Lenten sermons in which he condemned the immoral activities of the King through the example of King David's adultery—and the pious girl's conscience was troubled.[5] She fled to the convent at Chaillot. Louis followed her there and convinced her to return to court. Her enemies—chief among them, Olympe Mancini, comtesse de Soissons, niece of Cardinal Mazarin—sought to orchestrate her downfall by bringing her liaison to the ears of Louis's queen, Maria Theresa of Spain.
During her first pregnancy, Louise was removed from the Princess' service and established in a lodging in the Palais Royal, where, on 19 December 1663, she gave birth to a son, Charles, who was taken immediately to Saint-Leu and given to two faithful servants of Jean-Baptiste Colbert. Despite the secrecy of the transfer, organised by a doctor Boucher who was present at the birth, the story quickly spread to Paris. The public scorn at a midnight mass on 24 December resulted in a distraught Louise escaping home from the church.
Concealment was practically abandoned after her return to court, and within a week of Anne of Austria's death on 20 January 1666, La Vallière appeared at Mass beside Maria Theresa. Ashamed of her conduct, she treated the queen with humility and respect. In return, the queen was reportedly venomous towards her during the five-year affair, continuing even after the affair really ended —unaware that the king had taken another mistress.
After five years, Louise's favour was waning. On 7 January 1665 she had given birth to a second son, Philippe, and on 27 December of that year she gave birth a third son, Louis; but the three children soon died, Charles on 15 July 1665, Philippe before the autumn of 1666 and Louis shortly after. A daughter was born at Vincennes on 2 October 1666. In May 1667, by letters patent confirmed by the Parlement de Paris, Louis XIV legitimised his daughter, who was named Marie Anne de Bourbon and was given the title of Mademoiselle de Blois. Louis XIV also made Louise a duchess and gave her the estate of Vaujours. As a duchess, Louise had the right to sit on a tabouret in the presence of the queen, which was a highly prized privilege. However, Louise was not impressed. She said her title seemed a kind of present given to a servant who was retiring. Indeed, she was correct, for Louis commented that legitimising their daughter and giving Louise an establishment "matched the affection he had had for her for six years": in other words, an extravagant farewell present.
On 2 October of that year, she gave birth to their fifth child, a son named Louis, but by this time her place in the King's affections had been usurped by Françoise-Athénaïs, marquise de Montespan, whom both she and the queen (both pregnant when the affair began) had thought of as a trusted friend. Under the pretense of her pregnancy, Louise was sent away to Versailles while the King and the court were at the scene of the war; however, she disobeyed the King's orders and returned, throwing herself at his feet sobbing uncontrollably. In a strange twist of fate, she ended her relationship with the King in the same way in which she started: used initially as a decoy for Louis and "Madame", Louise now became a decoy for her own successor, as Louis made her share the Marquise de Montespan's apartments at the Tuileries to prevent the legal manœuvres of the Marquis de Montespan (who wanted to get his wife back) and to keep the court from gossiping.
Mlle de La Vallière was the godmother of Athénaïs' and Louis XIV's first daughter, who was given the first name Louise. Louise hated being the decoy for Athénaïs and begged and wept often to be allowed to join a convent. She attempted to leave in 1671, fleeing to the convent of Ste Marie de Chaillot, only to be compelled (once more by order of the King) to return. In 1674, she was finally permitted to enter the Carmelite convent in the Faubourg Saint-Jacques in Paris under the name of Sister Louise of Mercy.