In November 1924 he trained at the “Leninist school” in Bobigny, together with Jacques Duclos, and stood out for his ability to write for union and political newspapers.
[2] Suspected of Trotskyism in 1930 because of his friendship with Albert Cornette, secretary of the CGTU, he lived a brief period of misfortune until 1932 when he resumed responsibilities in the North, then as a permanent member of the Central Committee, in Paris, where he was responsible for directing the Association of Friends of the Soviet Union (AUS), a position he held until 1939.
In 1935, Fernand Grenier was responsible for the reconquest of the city of Saint-Denis, then in the Seine department, whose mayor, Jacques Doriot, had just been excluded from the Party.
He then met Colonel Rémy on November 25, 1942, and with him arrived in England in January 1943 carrying a letter from the CC of the Communist Party and another from Charles Tillon, leader of the Francs-Tireurs et Partisans (FTPF).
In September 1950, together with Denise Ginollin, he went to the Ministry of the Interior to inquire Eugène Thomas for clarification about the ban on certain books and magazines from the USSR as well as the newspaper of the Unified Socialists of Catalonia Lluita.