Guérin joined Pierre Monatte‘s revolutionary syndicalist group in 1930. He rejoined the SFIO in October 1935 to work with Marceau Pivert ‘Left Revolutionary’ tendency. In 1937 he organised a meeting opposing colonial repression with speakers from Algeria, Morocco, Indochina and Tunisia ( Habib Bourguiba ).
In February and March 1939 Guérin corresponded with Trotsky, although he disagreed with the declaration of the Fourth International.
In September 1960 Guérin and his wife Anne were among the first signatories of the ‘121 Call’ by artists, musicians and writers to recognise the Algerian war as a legitimate struggle for independence, demanding conscientious objectors be supported. Other signatories included De Beauvoir, Sartre, Sagan, Boulez, Rosmer, Breton, Truffaut, Lefebvre and Signoret.
in 1963 he wrote a report for Ben Bella on workers’ control in Algerian businsses, and after Ben Bella was overthrown in June 1965 he helped found a committee defending him and other victims of the subsequent repression.
In 1965 he published a book on Anarchism that sold thousands
Having written about his sexuality in the 1950s, in 1968 he became known as the ‘grand-father’ of the gay rights movement in France.
In 1969 he joined the Libertarian Communist Movement just founded by Georges Fontenis. In the 1970s he increasingly was interested by the ideas of Rosa Luxemburg.