Magnussen started figure skating at the age of six. Your first trainer was Hellmut May . At the age of twelve she became Canadian figure skating champion in the juniors. Her strengths quickly became apparent in the freestyle. In 1967 she was national runner-up in the seniors behind Valerie Jones . In the same year she played her first world championship and finished it in twelfth place. In the Olympic year 1968 Magnussen was the first Canadian champion in the seniors. At the World Championships in Geneva as well as at the Olympic Games in Grenoble , she finished seventh. In 1969 she faced setbacks. First she lost the Canadian Championship to Linda Carbonetto before she was diagnosed with fatigue fractures in both legs a few weeks later. This made participation in the World Cup impossible. She had to spend weeks in a wheelchair with both legs in a cast. During this time, she thought of retiring, but decided to continue. By 1970 she was back and better than ever. She became Canadian champion, a title she was to defend until 1973, and with fourth place at the world championship just missed a medal against the American Julie Lynn Holmes . At the world championship in 1971 in Lyon Magnussen was able to win her first world championship medal with bronze behind Beatrix Schuba and Holmes.
Like her rival, Janet Lynn from the USA, Magnussen had her strengths in the freestyle. At that time, however, the freestyle evaluation had a low weighting in contrast to the compulsory figure evaluation and thus both had no chance against the probably best compulsory figure runner in history, Beatrix Schuba from Austria. Schuba won gold at the 1972 Olympic Games in Sapporo , although she was only seventh in the freestyle, Lynn and Magnussen, who came first and second in the freestyle, won bronze and silver, respectively. Confronted with the dissatisfaction of the audience with the scoring system, the International Skating Union reduced the weighting of the compulsory figures by introducing a short program from 1973. This and the resignation of Schuba motivated the strong freestyle skiers Magnussen and Lynn to continue. At the World Championships in Bratislava Magnussen ran an almost flawless short program, while Lynn surprisingly fell twice. The victory in the freestyle before Magnussen was no longer enough for Lynn and so Karen Magnussen became world champion with a unanimous judge's verdict , the last Canadian world champion in figure skating to this day.
After the end of her amateur career in 1973, she switched to the professionals and ran for the ice revue Ice Capades until 1977. She then became a trainer in Boston and later in her hometown of Vancouver. She established a foundation that aims to enable talented young figure skaters to take up professional training at affordable prices.