He was born at Buhy in Normandy. His mother had leanings toward Protestantism, but his father tried to counteract her influence by sending him to the College de Lisieux at Paris. On his father's death in 1559, however, the family formally adopted the reformed faith. Mornay studied law and jurisprudence at the University of Heidelberg in 1565 and the following year Hebrew and German at the University of Padua. On the outbreak of the second religious war in 1567, he joined the army of Condé, but a fall from his horse prevented him from taking an active part in the campaign. His career as Huguenot apologist began in 1571 with the work Dissertation sur l'Église visible, and as a diplomat in 1572 when he undertook a confidential mission for Admiral de Coligny to William the Silent, prince of Orange.
He escaped the St. Bartholomew's Day Massacre by the aid of a Catholic friend, and took refuge in England. Returning to France towards the end of 1573, he participated during the next two years with various success in the campaigns of Henry IV of France, then King of Navarre only. He was taken prisoner by the Duke of Guise on the October 10, 1575, but ransomed for a small sum. Shortly afterwards he married Charlotte Arbaleste at Sedan. Mornay was gradually recognized as Henry's right-hand man, representing him in England from 1577 to 1578 and again in 1580, and in the Low Countries 1581-1582. With the death of the Duke of Alençon-Anjou in 1584, by which Henry was brought within sight of the throne of France, the period of Mornay's greatest political activity began, and after the death of the Prince of Condé in 1588 his influence became so great that he was popularly styled the "Huguenot pope". He was present at the siege of Dieppe, fought at Ivry, and was at the siege of Rouen in 1591-92, until sent on a mission to the court of Queen Elizabeth. He was bitterly disappointed by Henry IV's abjuration of Protestantism in 1593, and gradually withdrew from the court, devoting himself to university at Saumur, which had a distinguished history until its suppression by Louis XIV in 1683.
In 1598 he published a work on which he had long been engaged, entitled De L'institution, usage et doctrine du saint sacrement de l'eucharistie en l'église ancienne, containing about 5000 citations from the scriptures, fathers and schoolmen. Jacques Davy Du Perron, bishop of Évreux, afterwards cardinal and archbishop of Sens, accused him of misquoting at least 500, and a public disputation was held at Fontainebleau on May 4 1600. Decision was awarded to Du Perron on nine points presented, when the disputation was interrupted by the illness of Mornay.
His last years were saddened by the loss of his only son in 1605 and of his devoted wife in 1606, but he spent them in perfecting the Huguenot organization. He was chosen a deputy in 1618 to represent the French Protestants at the Synod of Dordrecht, and though prohibited from attending by Louis XIII, he contributed materially to its deliberations by written communications. He lost the governorship of Saumur at the time of the Huguenot insurrection in 1621, and died in retirement on his estate of La Forêt-sur-Sèvre (Deux-Sèvres).
His principal works, in addition to those mentioned above, are Excellent discours de la vie et de la mort (London, 1577), a bridal present to Charlotte Arbaleste; Traité de l'Église où l'on traite des principales questions qui ont été mues sur ce point en nostre temps (London, 1578); Vindiciae Contra Tyrannos in Latin under the pseudonym, Junius Brutus (1579); Traité de la vérité de la religion chrétienne contre les athées, épicuriens, payens, juifs, mahométans et autres infidèles (Antwerp, 1581); Le mystère d'iniquité, c'est à dire, l'histoire de la papauté (Geneva, 1611). Two volumes of Mémoires, from 1572 to 1589, appeared at La Forêt (1624-1625), and a continuation in 2 vols. at Amsterdam (1652); a more complete but very inaccurate edition (Mémoires, correspondances, et vie) in 12 vols. was published at Paris in 1624-1625. He is also one candidate for being author of the Vindiciae contra tyrannos (1579), a pamphlet advocating resistance to the French crown.
See the life of Mornay written by his wife for the instruction of their son, Mémoires de Mme Duplessis-Mornay, voi. i. in the ed. of Mémoires et correspondences de Duplessis-Mornay (Paris, 1824-1825); E. and E. Haag, La France protestante, article "Mornay"; J Ambert, Du Plessis-Mornay (Paris, 1847); E Stahelin, Der Übertritt K. Heinrichs IV. von Frankreich zur katholischen Kirche (Basel, 1856); Weiss, Du Plessis Mornay comme théologien (Strassburg, 1867). There is a good article "Du Plessis-Mornay" by T Schott in Hauck's Realencyklopädie, and another by Grube in Kirchenlexikon.