Of Dalmatian Italian and Croatian parentage, Garko was born Giovanni Garcovich in Zara (now Zadar, Croatia), at the time a part of Fascist Italy.

He continued to play parts in several Italian productions including sword and sandal epics such as The Mongols [fr] (1961) and Mole Men Against the Son of Hercules (1961).

Other notable westerns Garko starred in were $10,000 Blood Money (1967) as an unofficial Django, Vengeance is Mine/$100,000 for a Massacre (1967), The Price of Death (1971), They Call Him Cemetery (1971) alongside William Berger as well as a supporting role in Bad Man's River (1971) with Lee Van Cleef.

Outside of the western genre, Garko starred in Five for Hell (1969) with frequent co-star Klaus Kinski, a small role in Waterloo (1970) as the brave and energetic French artillery commander Antoine Drouot, and The Heroes (1973), both with Rod Steiger.

Among these are Cold Eyes of Fear (1971), The Night of the Devils (1972), Il Boss (1973), The Flower with the Petals of Steel (1973), Sette note in nero (1977 with Jennifer O'Neill), Joy of Flying (1977), Graf Dracula in Oberbayern (1979, a Bavarian sex comedy), Star Odyssey (1979), Encounters in the Deep (1979), Hercules (1983 with Lou Ferrigno), and Monster Shark (1984).