Klaes Karppinen won his first medal at the Finnish Championships, a silver in the 17 km, in 1934, and that would remain his only Finnish Championships medal until he won the Finnish title in the 1941 relay race. But Karppinen was much more successful internationally, making his international début at the 1934 World Championships, where he won a gold as a member of Finnish relay team and was fifth in the 18 km. The 1935 World Championships were successful for Karppinen, winning gold in both the 18 km and the relay, and a silver over 50 km. At the 1936 Winter Olympics Karppinen was a member of the victorious Finnish relay team and five days later placed fifth in the 50 km. At the 1937 World Championships, Karppinen was less successful, winning silvers in the 50 km and relay, but at the next 1938 World Championships, he won his third relay title while placing fifth in the 50 km. Karppinen made his last international appearance at the 1939 World Championships, where he won his fourth relay title and silvers at both 18 and 50 km. After his sporting career ended, Karppinen worked as a farmer.