Castillo was born into a musical family; both of his parents were classical musicians and pedagogues, his father a violinist and his mother a pianist.
Early in his life Castillo defined his aspirations to pursue a career in music as his parents had done.
[1] Castillo was educated in Santiago and in 1963 he began to study classical guitar at the National Conservatorium of Music which would later become part of the University of Chile.
[1] Castillo made numerous recordings with some of the leading folklorists of the New Chilean Song movement which included some of the most influential musicians in Latin America, including Víctor Jara, Los Parras and Los Jaivas.
[2] Early on in his career Castillo began to collaborate musically with Victor Jara on many of his albums and they jointly composed a song that would go on to become one of Victor Jara's most famous songs Plegaria a un Labrador (Prayer to a labourer).