When the Spanish Civil War broke out in July 1936, Juan Guilloto participated in the assault of Cuartel de la Montaña, and the Battle of Guadarrama fought in the Guadarrama Mountain Range. He was one of the leaders of Fifth Regiment, becoming its commander from October 1936 onwards. He fought in Talavera de la Reina, Santa Olalla and Illescas (September 1936), the defense of Madrid and the Second battle of the Corunna Road, as well as in the Battle of Jarama (February 1937).