She was born in Lyon, an illegitimate child of the comtesse d'Albon, but was brought up as the daughter of Claude Lespinasse of Lyon. At the age of 16, on leaving her convent school, she became governess in the home of her mother's legitimate daughter, Madame de Vichy, who had married the brother of Marie Anne de Vichy-Chamrond, marquise du Deffand. Here Madame du Deffand made her acquaintance, and, recognizing her extraordinary gifts, persuaded her to come to Paris as her companion.
Julie moved into Madame du Deffand's apartments in Convent of St. Joseph in 1754, location of the glittering salon that attracted famous diplomats, great ladies, philosophers and politicians. The relationship lasted ten years until 1764, when Madame du Deffand became jealous of the younger woman's increasing influence and a violent quarrel resulted.
Mlle de Lespinasse then set up a salon of her own which was joined by many of the most brilliant members of Madame du Deffand's circle. Jean le Rond d'Alembert was one of the most assiduous of her friends and eventually came to live in her house. This arrangement ensured d'Alembert's comfort and lent influence to Mlle de Lespinasse's salon. Although she had neither beauty nor rank, her ability as a hostess made her get-togethers the most popular in Paris. She owes her distinction, however, not to her social success, but to circumstances which remained a secret during her lifetime, even from her closest friends.
Two volumes of Lettres published in 1809 displayed her as the victim of a passion of a rare intensity. In virtue of this ardent, intense quality, Charles Augustin Sainte-Beuve and other of her critics place her letters in the limited category to which belong the Latin letters of Héloise and the Letters of a Portuguese Nun.
Her Lettres were published by Mme de Guibert in 1809 and a spurious additional collection appeared in 1820. Modern editions include that of Eugène Asse (1876-1877). Lettres inédités de Mademoiselle de Lespinasse à Condorcet, à D'Alembert, à Guibert, au comte de Crillon, edited by M. Charles Henry (1887), contains copies of the documents available for her biography. In addition to the Lettres she was the author of two chapters intended as a kind of sequel to Laurence Sterne's Sentimental Journey. Mary Augusta Ward's novel, Lady Rose's Daughter, owes something to the character of Mlle de Lespinasse.