Hermann was born in 1938 in Bévercé (now a part of Malmedy) in Liège Province. After studying to become a furniture maker and working as interior architect, Hermann made his debut as comic book artist in 1964 in the Franco-Belgian comics magazine Spirou with a four-page story. Greg noticed his talent and offered him to work for his studio. In 1966, he began illustrating the Bernard Prince series written by Greg, published in Tintin magazine. In 1969, also in collaboration with Greg, he began the western series Comanche. This appeared at the same time as other western series such as Blueberry.
Hermann began writing his own stories in 1977, starting the post-apocalyptic Jeremiah series, which is still produced today. In the same period, he also made three albums of Nick, inspired by Little Nemo in Slumberland, for Spirou. In 1983 he began a new series, Les Tours de Bois-Maury, which is set in the Middle Ages and is less focused on action than his other works.
Hermann has also created many non-series graphic novels sometimes together with his son Yves H. One of them, Lune de Guerre, with a story by Jean Van Hamme, was later filmed as The Wedding Party by Dominique Deruddere.
Hermann is characterized by a realistic style and stories that are both somber and angry, with a sense of disillusion with regards to the human character in general, and current society more specifically.