After taking his degree in Holy Theology in Toledo and studying Philosophy in Madrid, he worked at the Secondary School of Ourense.
Juan Sieiro collaborated with many articles in newspapers and magazines and published the following books: Lecciones de Filosofía, Psicología y Lógica (1873), Principios de Psicología o Antropología psíquica, Lógica y Ética (1882), Historia y Progreso de la Filosofía. Primera Parte. Analítica (1882), Programa razonado de Metafísica (1884) and Sociología (1888).