Born with the surname Pérez Guerra, he spent his childhood in Galicia before his mother moved to Madrid. In this city, and during his student years, he became acquainted with leading figures of Galician politics (for example Castelao) and the writers of the Generation of 27 of Spanish literature. He befriended Federico García Lorca, and played an important role in the elaboration of his Seis Poemas Galegos. In the Spanish Civil War, affected by the death of his friend Lorca, he enlisted in the Galician Militias of the Republican side. He was also a member of the Military Intelligence Service, on whose behalf he traveled to New York to investigate an alleged misappropriation of funds raised for the Republic. In Miguel Anxo Seixas Seoane’s version, Ramón Mosteiro (from the Frente Popular Antifascista Gallego) traveled to Barcelona to ask Pérez Guerra to join pro-Republican propaganda and the support to refugees in the EEUU. The end of the civil war found him in this city, where he lived for more than forty years.
He was given US citizenship on April 18, 1945 and changed his surname to Guerra da Cal. In New York, he developed an important academic career. He finished a doctorate in Portuguese literature with a thesis on Eça de Queiroz, worked as University Professor at New York University and City University of New York, and held important positions in the Association of Teachers of Spanish and Portuguese, the Modern Language Association of America and the National Selecting Committee for Fulbright Awards to Spain and Portugal. Guerra da Cal pioneered the development of Portuguese Studies in the United States. In addition to his academic work, he achieved fame as a poet (adopting the Portuguese spelling in his works also in a pioneering manner), with volumes such as Lua de alén mar (1959) and Rio de sonho e tempo (1962). Despite his academic and literary achievements, he is still a little-known figure in Galicia (for more information see the work of Joel R. Gômez, for example Ernesto Guerra da Cal, do exilio a galego universal (2015); Miguel Anxo Seixas Seoane, Castelao. Construtor da nación, tomo II (1931-1939) (2020)).