In 1860, Montmasson travelled to Sicily in preparation for the arrival of Rosolino Pilo and Giuseppe Garibaldi, who planned to start an uprising against the Bourbon-ruled Kingdom of the Two Sicilies. Montmasson moved throughout the countryside, gathering support for the imminent uprising. She then returned to Genoa in time for the Expedition of the Thousand, thus becoming the only woman to openly join the expedition's forces. During the ensuing fighting, she provided valuable intelligence on the island of Sicily and served as a nurse, earning the nickname of the "Angelo di Calatafimi" after the Battle of Calatafimi.