Flechtheim taught at institutions such as the Free University of Berlin, where he made his mark as one of the lecturers in office during the student movement. Without becoming involved in any particular party for any length of time, the politically active academic was always on the side of the left wing. On 5 March 2009, he would have been 100 years old.

Ossip K. Flechtheim was born on 5 March 1909 in Nikolayev, near Odessa; he came from a Westphalian Jewish family whose more well-known members included art dealer Alfred Flechtheim. Flechtheim arrived in Munich in 1911, then in 1920 went to Düsseldorf, where he obtained his university matriculation qualification (Abitur) in 1927. He subsequently began studying law and political science under teachers including Edmund Husserl, Alfred Weber, Karl Mannheim and Gustav Radbruch, eventually passing the first state law examination in Düsseldorf in 1931 and obtaining a doctorate in Cologne in 1934. Flechtheim joined the KPD when he was 18, but left the party six years later in 1932 because of its Stalinist politics. Soon after the Nazis came to power in January 1933, Flechtheim was dismissed from the civil service because of his political activism and Jewish origins; until his emigration, he worked underground for the left-wing socialist group “Neu Beginnen” led by Walter Löwenheim and continued to support them while he was in exile. He was arrested in 1935 but was able to flee to Belgium after his release. From the end of 1935 until his emigration to the United States in 1939, Flechtheim lived in Switzerland, where he studied at institutions such as the Institut des Hautes Etudes Internationales in Geneva. While he was in the USA (1939–1945 and 1947–1951), he initially worked as an assistant at Max Horkheimer’s Institute of Social Research in New York and later as a lecturer and professor at various American colleges.

Flechtheim first returned to Germany in June 1945, remaining there until August 1945 to hold a summer course on problems of government at the University of Heidelberg. One year later, he attended the Nuremberg war trials during the course of his work for the Office of the US Chief of Counsel for War Crimes, Robert M.W. Kempner. In September 1947, he was awarded the degree of Dr. phil. in Heidelberg. In 1952, he was appointed to a professorial post at the newly re-established Deutsche Hochschule für Politik (German College of Politics) in Berlin. In 1959 he became an associate professor and in 1961 a full professor of political science at the Otto Suhr Institute of the Free University of Berlin, where he taught the history and theory of parties until 1974. In 1962, Flechtheim left the SPD, which he had joined in 1952. In 1967, he joined with Hans Magnus Enzensberger and others to found the “Republican Club” in Berlin, which was to become a forum for the independent left wing. On 4 March 1998, Ossip K. Flechtheim died in Kleinmachnow, near Berlin. In 2003, the Humanistische Verband Deutschlands (German Humanist Federation) endowed the Ossip K. Flechtheim Prize in his honour; this is awarded every two years for outstanding commitment to “promoting awareness, tolerance and autonomy”.

Flechtheim's numerous academic publications include the dissertation Hegels Strafrechtstheorie (Hegel’s Theory of Crime and Punishment, Brno, 1936), Die KPD in der Weimarer Republik (The KPD in the Weimar Republic, Offenbach, 1948), Fundamentals of Political Science (New York, 1952; German version 1958), Von Hegel zu Kelsen (From Hegel to Kelsen, Berlin, 1963), Futurologie. Der Kampf um die Zukunft (Futurology. The Struggle for the Future, Cologne, 1970), Die Parteien der Bundesrepublik Deutschland (The Parties of the Federal Republic of Germany, Hamburg, 1973) and the autobiographical work Ausschau halten nach einer besseren Welt (Keeping Watch for a Better World, Berlin 1991).

Flechtheim's numerous correspondents included Willy Brandt, Ruth Fischer, Erich Fromm, Theodor Heuss, Gustav Heinemann, Richard Löwenthal, Golo Mann, Hans Mayer, Helmut Schmidt and Richard von Weizsäcker. 10 letters to Flechtheim written by Thomas Mann between 1939 and 1945 have also survived. - In 2002, the German Exile Archive 1933–1945 also received the estate of Flechtheim's widow Lili Flechtheim-Faktor, one of the daughters of Emil Faktor, long-standing editor-in-chief of the Berliner Börsen-Courier; she herself worked as a translator.