At the age of 15, she married a man 39 years older, Louis de Brézé, seigneur d'Anet. He was a grandson of King Charles VII who served the court of King François I. She bore him two daughters, Françoise de Brézé (1518 - 1574) and Louise de Brézé (1521 – 1577). In 1524, her father was accused of treason as an accomplice of the rebellious Connétable de Bourbon. He was nearly executed and learned that his life had been spared by Francis I as his head was already on the block.
When Louis de Brézé died in 1531 in Anet, Diane took up black as her main colour of dress for the rest of her life, adding later some white and grey. Her keen interest in financial matters and legal shrewdness became apparent for the first time. She managed to retain her late husband’s emoluments as governor and grand-sénéchal of Normandy, taking herself the title of "sénéchale de Normandie". She challenged in court the obligation to return Louis de Bézé’s appanages to the royal domain. The king allowed her to enjoy the appanage's income "until the status of those lands has been totally clarified."
She was a keen hunter and sportswoman and kept a fit body well into middle-age.
In 1533 the future Henri II married Catherine de' Medici. There had been strong opposition to this alliance, the Medicis being no more than upstarts in the eyes of many in the French court. Louis de Brézé and Diane approved this choice. Diane and Catherine were actually kin, being both descendants of the La Tour d'Auvergne family. Indeed, to Catherine, Diane was an intrusive elder cousin as well as a rival. As the future royal couple remained childless, concerned by rumors of a possible repudiation of a queen she had in control, Diane made sure that Henri's visits to his wife's bedroom would be frequent. She was in charge of the education of their children until 1551; her daughter Françoise managed the queen's servants. While Henri and Catherine would eventually produce ten children together, and despite the occasional affair, Diane de Poitiers would remain Henri's lifelong companion, and for the next 25 years she would be the most powerful influence in his life. Based on allusions in their correspondence, it is generally believed that she became his mistress in 1538.
Remembered as a beautiful woman, she maintained her good looks well into her fifties, and her appearance was immortalized in art. Only two signed paintings by the great artist François Clouet are known to exist, one being a painting of Diane. The subject of that painting shows her seated nude in her bath. She sat for other paintings of the time , often topless or nude, other times in traditional poses. All seem to depict a vibrant and attractive woman.
When Francis I was still alive, Diane de Poitiers had to compete at the court with Anne de Pisseleu, the king's favorite. She managed to have her exiled on her lands upon Francis I's death (1547).
Diane possessed a sharp intellect and was so politically astute that King Henri II trusted her to write many of his official letters, and even to sign them jointly with the one name HenriDiane. Her confident maturity and loyalty to Henri II made her his most dependable ally in the court. Her position in the Court of the King was such that when Pope Paul III sent the new Queen Catherine the "Golden Rose", he did not forget to present the royal mistress Diane with a pearl necklace. Within a very short amount of time she wielded considerable power within the realm. In 1548 she received the prestigious title of Duchess of Valentinois, then in 1553 was made Duchesse d'Étampes.
The king's adoration for Diane caused a great deal of jealousy on the part of Queen Catherine, particularly when Henri entrusted Diane with the Crown Jewels of France, had the Château d'Anet built for her, and gave her the beautiful Château de Chenonceau, a piece of royal property that Catherine had wanted for herself. However, as long as the king lived, the Queen was powerless to change this.
Despite her holding such power with the king, her status depended on the king's welfare, and his remaining in power. In 1559, when Henri was critically wounded in a jousting tournament, Queen Catherine de' Medici took control, restricting access to him.
Although the king is alleged to have called out repeatedly for Diane, she was never summoned or admitted, and on his death, she was also not invited to the funeral. Immediately thereafter, Catherine de' Medici banished Diane from Chenonceau to the Château de Chaumont. She stayed there only a short time, and lived out her remaining years in her chateau in Anet, Eure-et-Loir, where she lived in comfort but obscurity.
She died at 67. In accordance with her wishes, and to provide a resting place for her, her daughter completed the funeral chapel built near the castle. During the French Revolution, her tomb was opened and her remains thrown into a mass grave.
In 1866 Georges Guiffrey published her correspondence.
Lana Turner played Diane de Poitiers in the 1956 film Diane. Diane is one of title characters in the Alexandre Dumas, père novel, The Two Dianas. She also appears as a character in Madame de Lafayette's novel, La Princesse de Clèves. She is also the main character in Diane Haeger's novel "Courtesan".