French diplomat who was one of the most outspoken and well-known publicists for the Protestant cause during the French Wars of Religion (1562–98).

Mornay received a Protestant education, studying Hebrew, law, and German at the University of Heidelberg. He only narrowly escaped death while in Paris at the time of the massacre of Protestants on St. Bartholomew's Day (Aug. 24, 1572). During the next four years he wrote numerous political tracts, including Discours au roi Charles (1572; “Discourse to King Charles”) and Remonstrances aux estats pour la paix (1576; “Remonstrances on the Conditions for Peace”). Scholars have disputed whether the Vindiciae contra tyrannos (1579; “A Defense of Liberty Against Tyrants”), the most famous tract of Protestant political thought of the time, should be attributed to Mornay or to his friend Hubert Languet. The Vindiciae acknowledges a contract between a sovereign and his people: if the sovereign becomes a tyrant, the contract is broken and the people have the right to depose him.

Fighting for the Huguenots, Mornay was captured in 1575, but by concealing his identity he was able to secure release for only a small ransom. In 1576 he married Charlotte Arbaleste, whose memoirs are a major source for the events of her husband's life. Mornay became a valued counsellor of Henry of Navarre (later King Henry IV of France) and negotiated the reconciliation between Navarre and Henry III of France in April 1589. He conducted many important embassies for the Protestant cause and for Henry IV both before and after Henry's accession to the throne. During this time he was appointed governor of Saumur.

Henry IV's reconciliation with the Roman Catholic church (1593) ended his collaboration with Mornay, and the publication of Mornay's De l'institution . . . de l'Eucharistie (1598), in which he made use of scriptural quotations in an attack on Roman Catholic eucharistic doctrine, increased the breach between them. At a public disputation at Fontainebleau in 1600 with Jacques Davy Duperron, bishop of Évreux, it became clear that Mornay had lost Henry IV's favour. He played no further part in national affairs and in 1621 was deprived of his governorship.

Mornay also wrote a history of the papacy (1611). His Mémoires et correspondance (collected ed., 12 vol., 1824–25) contains many documents of French Protestant policy.