Born in the family chateau at Verrières-le-Buisson, a suburb southwest of Paris, she was the descendant of a great French seed company fortune, that of Vilmorin. She was afflicted with a slight limp that became a personal trademark. Vilmorin was best known as a writer of delicate but mordant tales, often set in aristocratic or artistic milieus. Her most famous novel was Madame de, published in 1951, which was made into the celebrated film The Earrings of Madame de... (1953), starring Charles Boyer and Danielle Darrieux and directed by Max Ophüls. Vilmorin's other works included Juliette, La Lettre dans un taxi, Les Belles Amours, Saintes-Unefois, and Intimités. Her letters to Jean Cocteau were published after the deaths of both correspondents.
As a young woman, in 1923, she had been engaged to the novelist and aviator Antoine de Saint-Exupéry. Vilmorin's first husband was an American real-estate heir, Henry Leigh Hunt (1886–1972). They married in 1925, moved to Las Vegas, Nevada, where Hunt's family owned extensive properties, and divorced in the 1930s. They had three daughters: Jessie, Alexandra, and Helena.
Her second husband was Count Paul Pálffy ab Erdöd (1890–1968), a much-married Austrian-born Hungarian playboy, who had been second husband to the Hungarian countess better known as Etti Plesch, owner of two Epsom Derby winners. Palffy married Louise as his fifth wife in 1938, but the couple soon divorced.
Vilmorin was the mistress of another of Etti Plesch's husbands Graf Maria Thomas Paul Esterházy de Galántha (1901–1964), who left his wife in 1942 for Vilmorin. They never married. For a number of years, she was the mistress of Duff Cooper, the British ambassador to France. Louise spent the last years of her life as the companion of the French Cultural Affairs Minister and author André Malraux, calling herself "Marilyn Malraux".
Francis Poulenc nearly literally sang her praises, considering her an equal to Paul Éluard and Max Jacob, found in her writing "a sort of sensitive impertinence, libertinage, and appetite which carried on into song [is] what I tried to express in my extreme youth with Marie Laurencin in Les Biches." (Ivry 1996)
She had a limp but possessed an ethereal elegance. Evelyn Waugh described "Loulou" to Nancy Mitford as "an Hungarian countess who pretended to be a French poet. An egocentric maniac with the eyes of a witch. She is the Spirit of France. How I hate the French." Mitford concurred, "Oh how glad I am you feel this about Lulu—I can't sit in a room with her she makes me so nervous. And vicious… She is much more like a middle European than a French woman." (Ivry 1996)