Berneri, a World War I veteran, University of Florence professor of humanities, and a member of the Unione Anarchica Italiana, had been opposed to the takeover of his country by Fascists, engaging in resistance until 1926, when he was forced to take refuge in France, then Switzerland, Germany, Belgium, Luxembourg, and finally the Netherlands. In all of these countries, Berneri was frequently placed under arrest and usually expelled. Although under constant surveillance, he nonetheless wrote on several topics, mostly anti-clerical and anti-fascist articles.
With Carlo Rosselli, Berneri organized the first column of Italian volunteers to fight in the Spanish Civil War against Francisco Franco's attempted coup, where he took part in the battles of Monte Pelado and Huesca (in Aragon) together with the Spanish anarchists of Francisco Ascaso Column and the Italians of the Matteotti Battalion. He became highly critical of the involvement of Confederación Nacional del Trabajo (CNT) members in the Popular Front government, expressing his views in an open letter to Federica Montseny (Anarchist minister for Health).
During the Barcelona May Days, as squads of Communist Party of Spain members (apparently under orders from Joseph Stalin) took to the streets in order to hunt down leading anarchists, Berneri was dragged from his home and murdered. His body, riddled with bullets, was found during the night, near the headquarters of the Generalitat de Catalunya.