Mann joined the Nazi Party in 1931, two years before they came to power, and also joined the Sturmabteilung, achieving the rank of Sturmführer in the Brownshirts. At the time of his initial membership Mann was the only member of the Vorstand to hold a Nazi Party card.[ Mann's membership had lapsed after a year although he quickly renewed his subscription as soon as Adolf Hitler became Chancellor of Germany.Following the Nazi seizure of power Mann penned a letter which he sent out to 75 of IG Farben's leading international sales representatives across the world. In the letter Mann told the executives that the Nazis had "won a victory against Bolshevism, the enemy of the entire world" and instructed them to tell their clients in their host countries that stories circulating about Nazi repression contained "not a true word". Mann was arrested by the U.S. Army in 1945 and two years later was indicted as part of the IG Farben trial. There he faced the charges of plundering, spoliation and mass murder, but he was acquitted in 1948.The following year he returned to his earlier role of head of pharmaceutical sales at Bayer.He also returned to market research company GfK, of which he had been president from 1935 to 1945, continuing in this role until 1955, whilst also holding a role on the Foreign Trade Committee of the Bundesverband der Deutschen Industrie.