During the brief campaign of the Sardinian army on the south-eastern frontier of France in 1815 he served as captain of grenadiers, and was afterwards employed in the ministry of war. The revolutionary and imperial epoch had seen a great development of Italian patriotism, and Santarosa was aggrieved by the great extension given to the Austrian power in Italy in 1815, which reduced his own country to a position of inferiority. The revolutionary outbreak of 1820, which extended from Spain to Naples, seemed to afford the patriots an opportunity to secure the independence of Italy.
When in 1821 the Austrian army was moved south to coerce the Neapolitans, Santarosa entered into a conspiracy to obtain the intervention of the Piedmontese in favor of the Neapolitans by an attack on the Austrian lines of communication. The conspirators endeavoured to obtain the co-operation of the prince of Carignano, afterwards King Charles Albert, who was known to share their patriotic aspirations.
On March 6, 1821 Santarosa and three associates had an interview with the prince, and on March 10 they carried out the military pronunciamiento which proclaimed the Spanish constitution. The movement had no real popular support, and very soon collapsed. During the brief predominance of his party Santarosa showed great decision of character. He was arrested and would have died on the scaffold if sympathisers had not rescued him. He fled to France, and lived for a time in Paris under the name of Conti. Here be wrote in French and published in 1822 his La Revolution Piemontaise, which attracted the notice of Victor Cousin, by whom he was aided and concealed.
The French government discovered his hiding-place, and he was imprisoned and expelled from Paris. After a short stay first at Alençon and then in Bourges, he passed over to England, where he found refuge in London with the poet Ugo Foscolo, and made a few English friends. He went to Nottingham, in the hope of being able to support himself by teaching French and Italian. The miseries of exile rather than any hope of advantage led him to accompany his countryman Giacinto Collegno to Greece in November 1824. The Italians were ill-treated by the Greeks and were not well looked on by the Philhellene committees, who thought that their presence would offend the powers. Santarosa was killed, apparently because he was too miserable and desperate to care to save his life, when the Egyptian troops attacked the island of Sphacteria, near Navarino, on May 8, 1825.