Hjalmar began playing cello as a pupil for Aage Kvalbein's students at the Norwegian Academy of Music in Oslo. He later studied at the Guildhall School of Music and Drama in London with Professor Leonard Stehn. As a freelance cellist he has played with most of the orchestras in Norway and has since 1998 been employed at the Stavanger Symphony Orchestra, where he has been head of the orchestra's artistic council for many years.
Hjalmar is a member of the Zeuner Quartett with a number of performances at the Stavanger Chamber Music Festival. He has also been active as a county musicians in Troms (Northern Norway) and a participant at Risør Festival Strings.
Hjalmar has taught cello in Oslo and at Stavanger Kulturskole, lately mostly at summer courses in Valdres and Klækken in Norway.