Jennewein was born on 21 November 1919 at St Anton in the Tyrolean Alps, Austria. He became a world champion in the combined event in Zakopane in 1939, and received silver medals in slalom and in downhill. In 1941 Jennewein participated at the FIS Alpine World Ski Championships 1941 in Cortina d'Ampezzo, Italy and won gold medals in downhill and the combined event. In 1946, the results were cancelled by the FIS because of the limited number of participants from only German-friendly countries during World War II.