• Philosopher and Political Activist Claude Lefort, 1924
  

He was politically active by 1942 under the influence of his tutor, the phenomenologist Maurice Merleau - Ponty (whose posthumous publications Lefort later edited). By 1943 he was organizing a faction of the Trotskyist Parti Communiste Internationaliste at the Lycée Henri IV in Paris.

Lefort was impressed by Cornelius Castoriadis when he first met him. From 1946 he collaborated with him in the Chaulieu – Montal Tendency, so called from their pseudonyms Pierre Chaulieu (Castoriadis) and Claude Montal (Lefort). They published On the Regime and Against the Defense of the USSR, a critique of both the Soviet Union and its Trotskyist supporters. They suggested that the USSR was dominated by a social layer of bureaucrats, and that it consisted of a new kind of society as aggressive as Western European societies. By 1948, having tried to persuade other Trotskyists of their viewpoint, they broke away with about a dozen others and founded the libertarian socialist group Socialisme ou Barbarie. Lefort's text L'Expérience prolétarienne was important in shifting the group's focus towards forms of self - organization.

For a time Lefort wrote for both the Socialisme ou Barbarie journal and for Les Temps Modernes. His involvement in the latter journal ended after a published debate during 1952 - 54 over Sartre's article The Communists and Peace.

Lefort was for a long time uncomfortable with Socialisme ou Barbarie's "organizationalist" tendencies. In 1958 he, Henri Simon and others left and formed Informations et Liaison Ouvrières.

In his academic career, Lefort taught at the University of São Paulo, at the Sorbonne and at the École des Hautes Études en Sciences Sociales, being affiliated to the Centre de recherches politiques Raymond Aron. He has written on the early political writers Niccolò Machiavelli and Étienne de La Boétie and explored "the Totalitarian enterprise" in its "denial of social division... [and] of the difference between the order of power, the order of law and the order of knowledge".

Lefort studied at the Sorbonne. He became a Marxist in his youth under the influence of his teacher, Maurice Merleau-Ponty. From 1944, he belonged to the small French Trotskyite. In 1946, he met Cornelius Castoriadis who came to Paris from Greece. Right away, they formed a faction in the Trotskyist Parti Communiste Internationaliste called "Chaulieu – Montal Tendency", that left the party and became the Socialism or Barbarism group and which, in 1949, started a journal with this name.

Socialism or Barbarism considered the USSR to be an example of state capitalism and gave its support to anti-bureaucratic revolts in Eastern Europe — especially the uprising in Budapest in 1956. Differences of opinion brought about a schism within Socialism or Barbarism, and Lefort sided with Henri Simon, one of the founders of the groupInformations et Liaison Ouvrières (Workers' Information and Liaison) — later renamed "Informations et Correspondance Ouvrieres" (Worker's Information and Correspondence) — in 1958. That year he abandoned the idea and ideology of political revolution and ceased his militant activism.

After having worked amongst other places, in 1947 and 1948 for UNESCO, in 1949 Lefort passed the aggregation in philosophy: he taught at the high school in Nîmes (1950) and in Reims (1951). In 1951, he was recruited as a sociology assistant at the Sorbonne by Georges Gurvitch. In the year 1952 (following a dispute with Gurvitch), he was detached from the sociology section of the CNRS, until 1966, with a break of two years (1953 - 1954), when he was professor of philosophy at University of São Paulo (Brazil). As for the CNRS, the support of Raymond Aron led to his recruitment as a teacher of sociology at the University of Caen, where he worked from 1966 to 1971, the year when he defended as his doctoral thesis his book on Machiavelli, The Labour of Work. That same year, he was again hired as a researcher in the sociology section of the CNRS until 1976, when he joined the Ecole des Hautes Etudes en Sciences Sociales, where he stayed until his retirement in 1989.

The intellectual work of Lefort is strongly tied to his participation, often tension filled, in successive journals. With Les Temps Modernes ("Modern Times") – introduced by Merleau-Ponty – he took part in the "gatherings of collaborators" and wrote from 1945 until his debate with J. P. Sartre in 1953. In Socialism or Barbarism (which lasted from 1949 to 1967 and of which he was the co-founder), he was active until 1950, then from 1955 to 1958. He was involved in Textures (established in 1969) from 1971 to the end (1975) and there he brought in Castoriadis and Miguel Abensour. With them (as well as Pierre Clastres and Marcel Gauchet) he created Libre in 1977, which was published up until 1980, when there were some disagreements with Castoriadis as well as with Gauchet. From 1982 to 1984, he led Passé - Present where amongst others Miguel Abensour, Carlos Semprún Maura [fr], Claude Mouchard and Pierre Pachet participated. These last two as well as Claude Habib formed the reading committee of the Littérature et Politique that Lefort founded for the publisher Éditions Belin in 1987.

No doubt he assigned less importance to the research centers at which he had participated in EHESS: the CECMAS (center of the study of mass communication), founded by Georges Friedmann and which welcomed Edgar Morin, then the Centre Aron, which he frequented just before his death.

When Merleau-Ponty died in 1961, Lefort took charge of the publication of his manuscripts. In the 1970s, he developed an analysis of bureaucratic regimes of Eastern Europe. He read The Gulag Archipelago and published a book on Aleksandr Solzhenitsyn. His main ideas on Stalinist totalitarianism were published in 1981 in a collection titled L'Invention démocratique.