He was born at Fréjus in the south of France, and was educated for the church at the Sorbonne. While there, he eagerly imbibed the teachings of John Locke, Etienne Bonnot de Condillac, and other political thinkers, in preference to theology. Nevertheless he entered the church, and his learning and subtlety earned him rapid promotion to vicar-general and chancellor of the diocese of Chartres. In 1788 the proposed convocation of the States General of France after the interval of more than a century and a half, and the invitation of Necker to writers to state their views as to the constitution of the Estates, enabled Sieyès to publish his celebrated pamphlet, "What is the Third Estate?" He begins his answer: "Everything. What has it been hitherto in the political order? Nothing. What does it desire? To be something." For this mot he is said to have been indebted to Chamfort. The pamphlet was very successful, and its author, despite doubts about his clerical vocation, was elected as the last (the twentieth) of the deputies of Paris to the States General.
Despite his failure as a speaker, his influence was great; he strongly advised the constitution of the Estates in one chamber as the National Assembly, but he opposed the abolition of tithes and the confiscation of church lands. Elected to the special committee on the constitution, he opposed the right of "absolute veto" for the king, which Honoré Mirabeau unsuccessfully supported. For the most part, however, he kept quiet about his opinions in the National Assembly, speaking rarely and then generally with oracular brevity and ambiguity. He had considerable influence on the framing of the departmental system, but after the spring of 1790 he was eclipsed by others. Only once was he elected to the post of fortnightly president of the Constituent Assembly. Excluded from the Legislative Assembly by Robespierre's self-denying ordinance, he reappeared in the third National Assembly, known as the Convention (September 1792 - September 1795). There, his self-effacement was even more remarkable; it resulted partly from disgust, partly from timidity. He even abjured his faith at the time of the installation of the goddess of reason; and afterwards he characterized his conduct during the reign of terror in the ironical phrase, J'ai vécu ("I lived.") He voted for the death of King Louis XVI, but not in the contemptuous terms sometimes ascribed to him. He is known to have disapproved of many of the provisions of the constitutions of the years 1791 and 1793, but did little to improve them.
in 1795 he went on a diplomatic mission to the Hague, and was instrumental in drawing up a treaty between the French and Bataviann republics. He dissented from the constitution of 1795 (that of the Directory) in some important particulars, and refused to serve as a Director of the Republic. In May 1798 he went as the plenipotentiary of France to the court of Berlin in order to try to induce Prussia to make common cause with France against the Second Coalition. His conduct was skilful, but he failed in his main object. The prestige which encircled his name led to his being elected a Director of France in place of Jean-François Rewbell in May 1799.
Already he was plotting the overthrow of the Directory, and is said to have considered the advent to power at Paris of persons so unlikely as the Archduke Charles and the duke of Brunswick. He now set himself to undermine the constitution of 1795. With that aim he caused the revived Jacobin Club to be closed, and made overtures to General Joubert for a coup d'état. The death of Joubert at the Battle of Novi, and the return of Bonaparte from Egypt marred his schemes; but ultimately he came to an understanding with the young general. After the coup d'état of Brumaire, Sieyès produced the perfect constitution which he had long been planning, only to have it completely remodelled by Bonaparte.
Sieyès soon retired from the post of provisional consul, which he accepted after Brumaire; he now became one of the first senators, and rumour, probably rightly, connected this retirement with the acquisition of a fine estate at Crôsne. After the bomb outrage at the close of 1800 (the affair of Nivôse), Sieyès in the senate defended the arbitrary and illegal proceedings whereby Bonaparte rid himself of the leading Jacobins. During the empire he rarely emerged from his retirement, but at the time of the Bourbon restorations (1814 and 1815) he left France. After the July revolution (1830) he returned.
See A Neton, Sieyès (1748-1836) d'après documents inédits (Paris, 1900); also the chief histories on the French Revolution and the Napoleonic empire.