Born in the United States, he was studying Medicine in Santiago de Compostela when the Civil War began. According to Emilio González López, he immediately returned to the United States and became president of the Fronte Popular Antifascista Gallego. Elixio Villaverde García indicates that Mosteiro was in the Republican Zone in Autumn of 1938 ‘with material and moral aid’, and returned to New York with Ernesto Guerra da Cal, bringing a letter from Ramón Suárez Picallo, ‘To the brothers of the Frente Popular Antifascista Gallego of New York’. The letter, reproduced in Villaverde García’ work, confirms Mosteiro’s presence at the front (Sources: Elixio Villaverde García, “A forxa dun rebelde con causa: Ramón Suárez Picallo” (1993); Emilio González López, Castelao, propagandista da República en Norteamérica (2000)).