In October 1935, Diel and his classmates formed a student group called 'Regain', where they conducted a weekly review of the same name dedicated to discuss once or twice a month of contemporary creativity with different artists in the basement of the Capoulade Quartier.
In October 1943, during the Nazi Occupation, Diehl founded the May Salon in Paris in a café on the Palais Royal, in opposition to the ideology of Nazism and its condemnation of degenerate art.
Its other founder-members were Henri-Georges Adam, Emmanuel Auricoste, Lucien Coutaud, Robert Couturier, Jacques Despierre, Marcel Gili, Leon Gischia, Francis Gruber, René Iché, Jean Le Moal, Alfred Manessier, André Marchand, Edouard Pignon, Gustave Singier, Claude Venard and Roger Vieillard.
Diehl also helped with the exhibitions of Bonnard, Matisse, Picasso, Villon, Braque, Bernard Buffet and Hans Hartung, and thus, accordingly including the testimony of Pierre Restany that he "was very conscious writer who had a foot in the first half of his century and another in the second."
In February 1966, he directed the Bureau des Expositions de l'Action Artistique, Ministry of Foreign Affairs, and took until his retirement in 1977 the task of start with the help of curators, critics and friends of exhibitions in France at the Grand and Petit Palais and the Louvre (including The Treasury Toutankamon in 1960).