At an early age he was sent to study at Paris. After some time spent in dissipation he began to cultivate the society of the wits of the time, and became a member of the club or association of young men who published their joint efforts in light literature under the titles of Recueil de ces messieurs, Etrennes de la St-Jean, Oeufs de Pasues, etc. His romance of Acajou et Zirphile, composed to suit a series of plates which had been engraved for another work, was one of the fruits of this association, and was produced in consequence of a sort of wager amongst its members. Duclos had previously written two other romances, which were more favourably received--The Baroness de Luz (1741), and the Confessions of the Count de *** (1747).
His first serious publication was the History of Louis XI, which is dry and epigrammatical in style, but displays considerable powers of research and impartiality. The reputation of Duclos as an author was confirmed by the publication of his Considerations sur les mvurs de ce siècle (1751), a work justly praised by La Harpe, as containing a great deal of sound and ingenious reflection. It was translated into English and German. The Mémoires pour servir de l'histoire du dixhuitième siècle, intended by the author as a sort of sequel to the preceding work, are much inferior in style and matter, and are, in reality, little better than a kind of romance. In consequence of his History of Louis XI, he was appointed historiographer of France, when that place became vacant on Voltaire's retirement to Prussia. His Secret Memoirs of the Reigns of Louis XIV and Louis XV (for which he was able to utilize the Mémoires of Saint-Simon, suppressed in 1755), were not published until after the Revolution.
Duclos became a member of the Academy of Inscriptions in 1739, and of the Académie française in 1747, being appointed perpetual secretary In 1747 both academies were indebted to him not only for many valuable contributions, but also for several useful regulations and improvements. As a member of the Academy of Inscriptions, he composed several memoirs on trial by combat, on the origin and revolutions of the Celtic and French languages, and on scenic representations and the ancient drama. As a member of the French Academy, he assisted in compiling the new edition of the Dictionary, which was published in 1762; and he made some just and philosophical remarks on the Port Royal Grammar. On several occasions he distinguished himself by vindicating the honour and prerogatives of the societies to which he belonged, and the dignity of the literary character in general. He used to say of himself, "I shall leave behind me a name dear to literary men."
The citizens of Dinan, whose interests he always supported with zeal, appointed him mayor of their town in 1744, though he was resident at Paris, and in this capacity be took part in the assembly of the estates of Brittany. Upon the requisition of this body the king granted him letters of nobility. In 1763 he was advised to retire from France for some time, having rendered himself obnoxious to the government by the opinions he had expressed on the dispute between the duc d'Aiguillon and M. de la Chalotais, the friend and countryman of Duclos. Accordingly he set out first for England (1763), then for Italy (1766); and on his return he wrote his Considerations on Italy. He died at Paris on the 26th of March 1772.
The character of Duclos was singular in its union of impulsiveness and prudence. Rousseau described him very laconically as a man droit et adroit. In his manners he displayed a sort of bluntness in society, which frequently rendered him disagreeable; and his caustic wit on many occasions created enemies. To those who knew him, however, he was a pleasant companion. A considerable number of his bons mots have been preserved by his biographers.
A complete edition of the works of Duclos, including an unfinished autobiography, was published by Auger (1821). See also Sainte-Beuve, Causeries du lundi, t. ix.; René Kerviler, La Bretagne et l'Académie française du X Ville siècle (1889); L Mandon, De la valeur historique des mémoires secrets de Duclos (1872).