Seppo Reijonen started to compete in ski jumping when he was just six-years-old. In the 1960s Reijonen was part of the Finnish national team and was selected to participate at the 1968 Grenoble Olympics. At the Games he placed 40th on the large hill with 170.1 points from his two jumps. One year later he moved to Sweden, where he soon joined their national team. Throughout the 1970s Reijonen was a regular at the Four Hills Tournaments and the Ski Flying World Championships, and became the Swedish champion on the large hill in 1978. He retired from the sport in 1983, becoming the chairman of the Fagersta ski jumping club and helping to build new slopes and coach young athletes.