No biographical details are currently known.
I've been familiar with Juan for many years and had always assumed that he would be fairly well known, but with no profile on Enciclopedia del Cine Español and more than half his roles unidentified on IMDb, it would seem not. Web searches are hindered by him having an extremely common Spanish name.
His early career is rather sporadic. He has 2 fairly substantial roles in the early 1940's, followed by a 12 year gap before his next credited role in 1955. There is then a further 5-year gap before his next known roles. From the start of the 1960's through until 1974 he worked quite prolifically in small speaking roles, usually getting only a couple of lines of dialogue. Casting directors seem to have had him pinned as their first choice for playing police inspectors. After 1974 the trail goes cold.
I have found a few undocumented roles and it is highly likely that there will be more.
- Un alto en el camino .... Sebastián (credited)
1943 - Un caballero famoso .... Paquiro (credited)
1955 - Congreso en Sevilla .... Owner of the bodega (credited)
1960 - 091: Policía al habla .... Police inspector (credited)
- El príncipe encadenado .... Baron of Gunther (credited)
- El vagabundo y la estrella .... Police inspector (credited)
- La reina del Tabarín .... Arturo del Castillo (credited)
- Mi último tango .... Dr. Eladio Ferrer (credited)
- Navidades en junio .... Solitary seducer (credited)
- Un ángel tuvo la culpa .... Police inspector (credited)
- Un rayo de luz .... Doctor (credited)
- Y el cuerpo sigue aguantando .... Restaurant owner (credited)
1961 - Armas contra la ley .... Police inspector (credited)
- Bello recuerdo .... Police commissioner (credited)
- Cariño mío .... Officer escorting Veronica to the palace (credited)
- La IV carabela .... Man explaining folk music to the ambassador (uncredited)
- Mi noche de bodas .... Police inspector (credited)
- Siempre es domingo .... Jeweller (credited)
- Tres de la Cruz Roja .... Barber's customer (credited)
- Usted puede ser un asesino .... Insurance agent (credited)
1962 - Aprendiendo a morir .... Police commissioner (credited)
- El balcón de la Luna .... Television presenter (credited)
- El grano de mostaza .... Doctor (credited)
- El hijo del capitán Blood .... Sir James McBride (credited)
- El hombre del expreso de Oriente .... Mayor (credited)
- Escuela de seductoras .... Belgian delegate (credited)
- I sette gladiatori .... The Emperor (credited as "Antonio Cortes")
- I tromboni di Fra Diavolo …. (credited, but not found)
- La banda de los ocho .... Director (credited)
- La gran familia .... Police inspector (uncredited)
- La mentira tiene cabellos rojos .... Police inspector (credited)
- Martes y trece .... Gangster (credited)
- Mentirosa .... Police inspector (credited)
- Sabían demasiado .... Bank manager (credited)
- Teresa de Jesús .... Inquisitor arresting Teresa (credited)
- Una isla con tomate .... Don Felipe (credited)
- Vampiresas 1930 .... Police inspector (credited)
- Ventolera .... role unknown (credited)
1963 - Chantaje a un torero .... Don Felipe (credited)
- Cuatro bodas y pico .... Don José's boss (credited)
- Eva 63 .... Doctor (credited)
- La casta Susana .... Director of the theatre (uncredited)
- La pandilla de los once .... Police inspector (credited)
- La Tela de araña .... Man with the plans (uncredited)
- Las hijas de Helena .... Police officer (credited)
- Los conquistadores del Pacífico .... Quesada (credited)
- Operación Embajada .... Man at the club (credited)
1964 - Cuatro balazos .... Nick (credited)
- El salario del crimen .... Police inspector (credited)
- I promessi sposi .... role unknown (credited)
- Júrame .... role unknown (poster credit)
- La boda .... role unknown (uncredited)
- La muerte silba un blues .... Friend of Lina (credited)
- Le pistole non discutono .... Johnny Parker (uncredited)
- Per un pugno di dollari .... Cavalry Captain (uncredited)
- Relevo para un pistolero .... (credit only)
1965 - 7 uomini d'oro .... Metal works foreman (credited as "Juan Cortez")
- Adiós, gringo .... Judge Blackburn (credited as "Clyde Geryll", maybe)
- Aventuras del Oeste .... Mortimer (credited)
- Currito de la Cruz .... Don Enrique (credited)
- Dos chicas locas locas .... Ship's Captain (credited)
- El cálido verano del Sr. Rodríguez .... Don Miguel (credited)
- Historias de la televisión .... Game show judge (credited)
- Jandro .... Bank executive (credited)
- La carga de la policía montada .... Jackson (credited)
- La magnifica sfida .... Hussein (poster credit as "Juan Cortez")
- Los cuatreros .... Jed (credited)
- Marc Mato, agente S. 077 .... Head of the Secret Service (credited)
- Muere una mujer .... The Judge (credited)
- Operazione poker .... Sir Malcolm (credited as "Juan Cortez")
1966 - El arte de no casarse .... Military Sergeant (credited)
- Goldsnake 'Anonima Killers' .... Jean (credited)
- Las últimas horas... .... Miguel Maura (credited)
- Mestizo .... General (credited)
- Pochi dollari per Django .... Banker (uncredited)
- Proceso a una estrella .... Judge (credited)
1967 - ¿Qué hacemos con los hijos? .... Husband of the woman with the false pregnancy (uncredited)
- El castigo sin venganza (TV movie) .... Theatre patron (credit unknown)
- El hombre que mató a Billy el Niño .... Lincoln sheriff (uncredited)
- Joe l'implacabile .... Town Governor (uncredited)
- La familia Colón (TV series) Ep. 16: "Memorias de un perro" .... Businessman (uncredited)
- La niña del patio .... Colleague of Pedro (credited)
- Las cicatrices .... Bullfight organiser (uncredited)
- Los chicos del Preu .... Surgeon (credited)
1968 - Più tardi Claire, più tardi... .... Party guest (credited as "Juan Cortez")
1969 - Carola de día, Carola de noche .... Lawyer (uncredited)
- La última aventura del Zorro .... Captain Alvaro Mendoza (credited)
- Quinto: non ammazzare .... Town official (credited)
- Las 5 advertencias de Satanás .... Pedro (credited)
1970 - Los rebeldes de Arizona .... Railway businessman (credited)
- Plomo sobre Dallas .... Terry Morse (credited)
1971 - 20,000 dólares por un cadáver .... White (credited)
- Visto para sentencia (TV series) Eps. 2, 9 .... Chief Prosecutor (credited)
1972 - La noche del terror ciego .... Coroner (credited)
- Stop (TV series) Ep. 11: "Apuesta fatal" .... Doctor (credited)
1973 - Celos, amor y Mercado Común .... Hotel guest whose room is invaded (credited)
- Diálogos de carmelitas (TV movie) .... (credit only)
- Si yo fuera rico (TV series) Ep. 1: "Si yo fuera ejecutivo" .... Doctor (credited)
- Una muchachita de Valladolid (TV movie) .... Frutos (credited)
1974 - Carlo Monte en Montecarlo (TV movie) .... Rene (credited)
- Festival (1961) IMDb, as Publicity manager
- Escuela de maridos (1964 TV series) Ep.: "Lecciones de puntualidad", IMDb
- Fernández, punto y coma (1964 TV series) Ep. dated 09.02.1964. IMDb
- Teatro de familia (1964 TV series) Ep.: "La última puerta", IMDb
- Delitto d'amore, aka Abajo espera la muerte (1966) IMDb
- Solange du lebst (1955) IMDb as Moreno. This was a German production shot partly in Spain, but with an almost entirely German cast. I have checked the German print and he is not credited, nor does he appear as far as I could find. I suppose it could be possible that the Spanish print contains footage not found in the German, but IMDb does not even list a Spanish title to search for.
- El Zorro de Monterrey, aka Zorro la maschera della vendetta (1971) IMDb with no role. Not found by myself or Carlos.