Penttlä was a cross-country skier and shooter, and changed to javelin throw at the age of 15. He won the national title in 1926–1928, and set a new world record in October 1927. He was a favorite at the 1928 Olympics, but finished sixth due to an elbow injury. After that he lost domestic competitions to Matti Järvinen, and at the 1932 Games finished third. He retired in 1936 due to the old injury.