He was born in Larvik as the son of school inspector Hermann Olai Helle (1893–1973) and teacher Berta Marie Malm (1906–1991).
In October 1987 Helle married museum director and professor of medieval archaeology Ingvild Øye.
His main work was Konge og gode menn i norsk riksstyring ca.
[2] In historiography, he is known for reaching different conclusions than did important Norwegian historians of the 1930s, such as Edvard Bull, Sr. and Andreas Holmsen.
[2] In 2006 Helle published Norsk byhistorie: urbanisering gjennom 1300 år together with Finn-Einar Eliassen, Jan Eivind Myhre and Ola Svein Stugu.
[6] Helle was the vice dean of the Faculty of Humanities at the University of Bergen from 1978 to 1980, and chair of the Chr.