He was born in Vegadeo (Oviedo). From the administrative point of view, this town does not belong to Galicia but from the cultural point of view, it is considered Galician territory because of its language and folklore. Besides, Cotarelo Valledor contributed to Galician culture with many works.He got a degree in arts. In 1929, he was elected as a member of the Galician Academy together with Ramón Cabanillas. He also became a member of the Spanish Academy of History.
His best contribution to Galician Literature was his theatre production: Trebón (1922), Sinxebra (1923), Lubicán (1924), Hostia (1926), Beiramar (1931) e Mourenza (1931). His theatre plays characterise for the perfect structure of the plays and the great psychological description of the characters.He also wrote many other works both in Galician and Castilian language such as Contos de Nadal, colleitos do pobo (A Coruña, 1927), Cancioneiro da agulla enxergado con doas do pobo (A Coruña, 1931), La heráldica: Blasón galaico. Contribución al estudio de la heráldica gallega (A Coruña, 1928), Argonautas gallegos (1519-1522) (Santiago, 1920), Los hermanos Eans Mariño, poetas gallegos del siglo XIII (Madrid, 1933), Encol do nome de Martín Códax (Santiago, 1933), El castellano en Galicia (Madrid, 1927), and so forth.
The Galician Academy dedicated him ?O Día das Letras Galegas? (The Day of the Galcian Letters) in 1984.