In 1978 Mustonen was the only woman participant to represent Finland in the Alpine World Skiing Championships in Garmisch-Partenkirchen. She finished 40th in downhill and 44th in giant slalom. In slalom she dropped out in the second run. She competed in the 1978 Swedish Alpine Championships in Dundret, Gällivare, finishing third in giant slalom and fourth in slalom. Mustonen was voted by Finnish sports journalists as the best alpine skier in Finland in two consecutive years, 1977 and 1978.