A brilliant student, Deat graduated from the École Normale and taught philosophy in Reims. In 1926 he was elected to the Chamber of Deputies as a Socialist but broke with the party in 1932 in opposition to Léon Blum's leadership. During the 1930s, Déat steadily moved to the political right. Adopting the slogan “Order, Authority, and Nation,” he helped form the Parti Socialiste de France (July 1933) as an alternative to Blum's Socialist policies. After a brief term as air minister (1936) he lost his seat in the Chamber.

An outspoken admirer of Hitler's National Socialism, in April 1939 Déat caused a national controversy with his article “Mourir pour Dantzig?” (“Why Die for Danzig?”), which held that France had no interest in defending Poland against Hitler. When France fell to Germany (June 1940), he remained in the occupied zone and founded the collaborationist Rassemblement National Populaire. In March 1944 he joined Pierre Laval's government as minister of work and social affairs. After the collapse of Germany, Déat disappeared. In June 1945 he was condemned to death, in absentia, for treason. It was later discovered that a religious order in northern Italy had taken him in and concealed him until his death. During his last years, Déat wrote his autobiography, Mémoires politiques, which was published in 1989.