After working as a teacher for some time, he started writing in his early forties and published his first collection of stories, Vitner (Witnesses), in 1979.

Herbjørnsrud's playful mixing of dialects poses great challenges to translators; his most prolific story, Kai Sandemo (1997), has long been considered untranslatable to any other language.

It tells the story of a murderer who flees his home in Telemark and settles in Denmark where he starts writing letters to people he grew up with – first in Danish, then, as he remembers more and more dark secrets of his past, gradually switching to Nynorsk, Bokmål and the dialect of Telemark until at the end, virtually every sentence becomes a complex mixture of various languages and dialects.

An ambitious project undertaken by the universities of Zürich and Munich worked on a German translation of the story for several years and released a finished version in 2005.

Herbjørnsrud was married to Danish-born historian Anna Tranberg, former associate professor at Telemark University College in Bø.