He became a world champion in the combined event in Zakopane in 1939, and received silver medals in slalom and in downhill.

[1] 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.

To augment the remaining fighter units fighting on the Western Front, JFS 5 was tasked with creation of a fourth squadron named 4.

[4] That day, flying his fourth combat mission,[2] Jennewein claimed three Supermarine Spitfire fighters shot down near Fécamp.

[7] By the end of July 1942, when he was posted to serve as a flight instructor, he had added 12 Russian aircraft to his tally.

On 18 January 1943, as a Feldwebel, flying a Focke-Wulf Fw 190 as a wingman of Leutnant Joachim Brendel, he attacked a formation of nine Petlyakov Pe-2 bombers from 202 BAP, in the area of Velikiye Luki, and claimed five kills in five minutes (Brendel claimed three, actual Soviet losses were six).

[10] Shortly after, he was posted as missing in action when his Focke-Wulf Fw 190 A-6 (Werknummer 550182[Note 1] —factory number) was shot down by another Il-2 in aerial combat northwest of Mtsensk.

[12] Spick also lists Jennewein with 86 aerial victories, including five during the Battle of Britain and further 81 on the Eastern Front, claimed in 271 combat missions.