Harmstorf was the son of a doctor from Hamburg. He started a sports career and soon became a regional master of the decathlon. He then studied medicine, later music and performing arts. From the beginning of the 1960s he started performing in smaller TV productions. His breakthrough was in 1971 with the TV series The Sea-Wolf, based on Jack London's novel, where he played the evil-minded Captain Larsen. Later he played in several spaghetti westerns along with Bud Spencer, Franco Nero and Charlton Heston.
Toward the end of his career he was affected by Parkinson's disease and weakened by a regimen of heavy medication. His illness and vulnerability was greatly exploited by the tabloids. He committed suicide by hanging himself. His death caused a scandal.
German tabloids were investigated; German police consequently stated that Harmstorf's suicide had been substantially promoted by certain articles. In particular Bild was blamed because Bild  had already published Harmstorf's suicide on its main page before his actual death. Harmstorf's girlfriend confirmed that the actor had obviously already been dismayed after he had read this article, even before the news had been partly quoted on Germany's popular TV channel RTL Television.
The following is a selection of Harmstorf's roles in film: