She stayed at court after becoming a widow and, at age 35, became the mistress of the then 16-year-old Henri d'Orléans, later to be King Henri II. Although King Henri II fulfilled his duty by marrying the noble foreigner Catherine de Medici, Diane de Poitiers would remain his lifelong true love.
A beautiful woman who maintained her looks well into her fifties, Diane possessed an extreme intellect and a political astuteness to the point that the King trusted her to write many of his official letters and to even sign them jointly with the one name: HenriDiane. She was in fact, the “brains behind the throne,” and even in charge of the royal children’s education. 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 with a pearl necklace.
The king’s total 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 Chateau d'Anet built for her, and gave her the beautiful chateau at Chenonceaux that Catherine had wanted for herself.
In 1559, when Henri was critically wounded in a jousting tournament, Catherine de Medici took control, restricting access to him. Although the king called out repeatedly for Diane, she was never summoned, and on his death, she was also not invited to the funeral. Immediately thereafter, Catherine de Medici banished Diane from Chenonceaux to the Chateau Chaumont. She stayed there only a short time and lived out her remaining years in her chateau in Anet[?], Eure-et-Loir where she died.
In accordance with her wishes, and to provide a resting place for her, her daughter completed the funeral chapel built near the château.