Hagen, a regular on the 1960s TV adventure show Daktari, passed away on 7 May (11) in Brentwood, California after a battle with prostate cancer.
The actor made appearances on TV Westerns such as The Big Valley and The Virginian in the mid-1960s and also co-starred with Elvis Presley in Speedway.
He is best known for playing hunter Bart Jason in Daktari, about an animal study centre in Africa.
Hagen also went on to write and direct, helming Time Wars and The Media Madman.
He is survived by his partner, Lee Srednick, a son and a daughter, according to Variety.com.