As Mademoiselle Lange, Jeanne immediately became a sensation in Paris, building up a large aristocratic clientele. The dashing Maréchal de Richelieu became one of her recurring customers. Because of this, Jean du Barry saw her as a means of influence with Louis XV, who became aware of her in 1768 while she was on an errand at Versailles. In any case, Jeanne could not qualify as an official royal mistress unless she had a title; this was solved by her marriage on 1 September 1768 to du Barry's brother, comte Guillaume du Barry. The marriage ceremony included a false birth certificate created by Jean du Barry himself, making Jeanne younger by three years and of nobler descent.
For now, Jeanne was installed below the King's quarters in Lebel's former rooms. She lived a lonely life, not able to be seen with the King since no formal presentation had taken place. Her official sponsor, Madame de Béarn, presented her to the Court at Versailles on 22 April 1769. Jeanne was wearing a queenly silvery white gown brocaded with gold, bedecked in jewels sent by the king the night before, and with huge panniers at the sides. The dress had been ordered by Richelieu especially for Jeanne; many courtiers claimed that its likeness had never been seen before. Her hairdo was also noticeably spectacular, being the cause of her late arrival.
Jeanne was a tremendous triumph. She now wore extravagant gowns of great proportions both in creation and cost, exhausting the treasury all the more. With diamonds covering her delicate neck and ears, she was now the king's maîtresse déclarée. Due to her new position at Court, she made both friends and enemies. Her most bitter rival was the Duchesse Béatrix de Grammont, Choiseul's sister, who had in vain tried her best to acquire the place of the late Marquise de Pompadour. Jeanne's first friend was Claire Françoise, better known as 'Chon', brought from Languedoc by her brother Jean du Barry to accompany her then-friendless sister-in-law. Later on, she also befriended the Maréchale de Mirepoix and the Comtesse d'Ossun.
Jeanne quickly accustomed herself to living in luxury (which she had already been introduced to when living with Dumonceaux), having also been given a young Bengalese servant-boy by Louis XV, Zamor, whom she dressed in elegant clothing to show him off; but her good nature was not spoiled. When the old Comte and Comtesse de Lousene were forcibly evicted from their château due to heavy debts, they were sentenced to beheading because the Comtesse had shot dead a bailiff and a police officer while resisting. To their great fortune, they were good friends with Madame de Béarn, who told Jeanne of their situation. Though warned by Richelieu of her possible failure, she asked the king to pardon them, refusing to rise from her kneeling posture if he did not accept her request. Louis XV was astounded and his heart thawed, saying, "Madame, I am delighted that the first favour you should ask of me should be an act of mercy!".
While Jeanne was part of the faction that brought down the Duc de Choiseul, Minister of Foreign Affairs, she was unlike her late predecessor, Madame de Pompadour, in that she had little interest in politics, preferring rather to pass her time ordering new ravishing gowns and all sorts of complementary jewelery. However, the king went so far as to let her participate in state councils. A note in a modern edition of the Sovenirs of Mme. Campan recalls a pleasant anecdote: the king said to the duc de Noailles, that with Mme. du Barry he had discovered new pleasures; "Sire - answered the duke - that's because your Majesty has never been in a brothel."
While Jeanne was known for her good nature and support of artists, she grew increasingly unpopular because of the king's financial extravagance towards her. Her relationship with Marie Antoinette, who was married to the Dauphin of France, was contentious. Marie Antoinette supported Choiseul as the proponent of the alliance with Austria and also defied court protocol by refusing to speak to Mme. du Barry, owing not only to her disapproval of the latter's background, but also after hearing from the Comte de Provence of du Barry's amused reaction to a story told by the Prince de Rohan during one of her dinner parties, in which Marie Antoinette's mother, Maria Theresa, was slandered. Madame du Barry furiously complained to the king. Eventually, during a ball on New Year's Day 1772, Marie Antoinette spoke to her, saying, "There are many people at Versailles today," but made it clear that she would say nothing else to du Barry.
In 1772, the infatuated Louis XV requested that Parisian jewellers Boehmer & Bassenge create an elaborate and spectacular jeweled necklace for du Barry, one that would surpass all known others in grandeur, at an estimated cost of two million livres. The necklace, still not completed nor paid for when Louis XV died, would eventually trigger a scandal involving Jeanne de la Motte-Valois, in which Queen Marie Antoinette would be wrongly accused of bribing the Cardinal de Rohan, Archbishop of Strasbourg in the Alsace, to purchase it for her, accusations which would figure prominently in the onset of the French Revolution.
In time, the king started to show his age by constantly thinking of death and repentance, even missing 'appointments' in Jeanne's boudoir. During a stay at the Petit Trianon with her, Louis XV felt the first symptoms of smallpox. He was brought back to the palace at night and put to bed, where his three daughters and Madame du Barry stayed with him. On 4 May 1774, the king suggested to Madame du Barry that she leave Versailles, both to protect her from infection and to prepare for confession and receiving last rites. She immediately retired to her estate near Rueil. Following the death of Louis XV, she was quickly exiled to the Abbaye du Pont-aux-Dames near Meaux-en-Brie.