The son of a notary of Fréjus, Sieyès was educated for an ecclesiastical career at the Sorbonne and rose in the church to become vicar-general (1780) and chancellor (1788) of the diocese of Chartres. Nevertheless, because he was not of noble birth, his opportunities for further advancement in the church were limited. Hence, he was already embittered against the aristocracy by the time the States General were summoned in 1788. During the ensuing public controversy over the organization of the States General, Sieyès issued his pamphlet Qu'est-ce que le tiers état? (January 1789; “What Is the Third Estate?”), in which he identified the unprivileged Third Estate with the French nation and asserted that it alone had the right to draft a new constitution.

Although Sieyès enjoyed fame as a theorist, his vanity and lack of oratorical skill reduced his political effectiveness. He did vote with the majority of deputies for the king's execution (January 1793), but, when the radical democrats of the Jacobin Club seized control of the Revolution in June 1793 and launched the Reign of Terror, Sieyès withdrew from politics. Later he is said to have summarized his conduct during this period in the ironical remark “J'ai vécu” (“I remained alive”).

In 1795 Sieyès served for six months on the Committee of Public Safety, where he advocated an expansionist foreign policy. He was elected (October 1795) to the Council of Five Hundred set up under the republican constitution of 1795, and in May 1799 he won a seat on the five-member Directory, France's ruling executive board. Nevertheless, he had already concluded that the executive must be strengthened at the expense of the legislative bodies. Conspiring with General Napoleon Bonaparte (Napoleon I), Joseph Fouché, and C.M. de Talleyrand, he helped organize the military coup d'état that overthrew the Directory on 18 Brumaire (Nov. 9, 1799). The next day Sieyès, Bonaparte, and Pierre-Roger Ducos were made provisional consuls. A new constitution drafted by Sieyès provided for an elaborate balance of powers within the executive, but Bonaparte quickly altered the constitution to make himself first consul and supreme ruler of France. Thereafter, Sieyès' influence declined. He remained a senator and was nominated grand officer of the Legion of Honour (1804) and count of the empire (1808).

After the restoration of King Louis XVIII in 1815, Sieyès was banished as a regicide. He settled in Brussels but returned to Paris upon the overthrow of King Charles X in July 1830.