He was the eldest son of Lieutenant-General Count de Caylus. His mother, Marthe-Marguerite (née) Le Valois de Villette de Murçay, comtesse de Caylus (1673-1729), was a cousin of Mme de Maintenon, who brought her up like her own daughter. She wrote valuable memoirs of the court of Louis XIV entitled Souvenirs; these were edited by Voltaire (1770), and by many later editors, notably Renouard (1806), Ch. Asselineau (1860), M. de Lescure (1874), ME Rauni (1881), J Soury (1883).
While a young man Caylus distinguished himself in the campaigns of the French army, from 1709 to 1714. After the peace of Rastadt he spent some time in travelling in Italy, Greece, the East, England and Germany, and devoted much attention to the study and collection of antiquities. He became an active member of the Academy of Painting and Sculpture and of the Academy of Inscriptions. Among his antiquarian works are Recueil d'antiquités égyptiennes, étrusques, grecques, romaines et gauloises (6 vols., Paris, 1752-1755), Numismata Aurea Imperatorum Romanorum, and a Mémoire (1755) on the method of encaustic painting with wax mentioned by Pliny, which he claimed to have rediscovered.
Diderot, who was no friend to Caylus, maintained that the proper method had been found by JB Bachelier. Caylus was an admirable engraver, and copied many of the paintings of the great masters. He caused engravings to be made, at his own expense, of Bartoli's copies from ancient pictures and published Nouveaux sujets de peinture et de sculpture (1755) and Tableaux tires de l'Iliade, de l'Odyssée, et de l'Eneide (1757).
He encouraged artists whose reputations were still in the making, but his patronage was somewhat capricious. Diderot expressed this fact in an epigram in his Salon of 1765: "La mort nous a délivré du plus cruel des amateurs." Caylus had quite another side to his character. He had a thorough acquaintance with the gayest and most disreputable sides of Parisian life, and left a number of more or less witty stories dealing with it. These were collected (Amsterdam, 1787) as his Œuvres badines complètes. The best of them is the Histoire de M. Guillaurne, cocher (c. 1730).
The Souvenirs du comte de Caylus, published in 1805, is of very doubtful authenticity. See also A and J de Goncourt, Portraits intimes du XVIIIieme siècle; Ch. Nisard's edition of the Correspondance du comte de Caylus avec le père Paciaudi (1877); and a notice by O Uzanne prefixed to a volume of his Facties (1879).