Martinsen was first selected to compete at the international stage as a junior for Norway at the 2007 World Under-18 Championships. Finishing with a Division 1 Bronze medal, Martinsen was a fixture in the Norwegian junior team throughout his junior eligibility. Martinsen made his Senior international debut for Norway as a 20-year-old at the 2010 IIHF World Championships. He has appeared in each World Championships since, marking 7 consecutive Championships as of 2016.