He finished fifth in the 50 km event at the 1950 World Championships in Lake Placid, New York.
Maartmann also finished eighth in the 50 km event at the 1952 Winter Olympics in Oslo.
His grandfather was Harald Sigvart Maartmann, the first Executive Director of Ringnes.
His great-grandfather was Norwegian politician Knud Geelmuyden Fleischer Maartmann.
This biographical article relating to Norwegian cross-country skiing is a stub.