Before World War I, he was originally an individualist-anarchist and, together with his friend Benito Mussolini, collaborated on the socialist newspaper La lotta di classe.
The following year he was elected to the Chamber of Deputies and, after the March on Rome, he was made national vice-secretary of the Partito Nazionale Fascista (PNF).
In 1926 he intervened to strip FC Torino of the national football title after a corruption scandal; as the Bologna team had arrived second behind Torino, Arpinati, being a Bolognese, decided to leave the title unassigned (a feat unparalleled until the 2006 Calciopoli) to avoid accusations of personal interest.
In 1930 the PNF secretary Achille Starace accused Arpinati of being behind the attempt against Mussolini in the Bologna Stadium carried out on 31 October 1926.
In 1943 Arpinati refused a personal invitation by Mussolini to join the Repubblica di Salò, the German puppet-state created in northern Italy after the Allied conquest of the southern peninsula.