During his early years, Sixto Agudo joined the Socialist Youth. He was active during the peasants' general strike and the 1934 October Revolution. In April 1936 he became a member of the Toledo provincial committee of the Unified Socialist Youth. In the Spanish Civil War he fought at the battle fronts in Jarama, Madrid and Levante. He was captured by Italian troops and held in prison camps in Almendros, Albatera and Portacoeli. His last lieu of imprisonment was at Sa Pobla, on the island of Majorca. He managed to escape and reach France.