Fegelein joined a cavalry regiment of the Reichswehr in 1925 and transferred to the SS on 10 April 1933. He became a leader of an SS equestrian group, and was in charge of preparation for the equestrian events of the Berlin Olympic Games in 1936. He tried out for the Olympic equestrian team himself but was eliminated in the qualifying rounds.
In September 1939, after the Invasion of Poland, Fegelein commanded the SS Totenkopf Reiterstandarte (Death's-Head Horse Regiment). They were garrisoned in Warsaw until December. In May and June 1940, he participated in the Battle of Belgium and France as a member of the SS-Verfügungstruppe (later renamed the Waffen-SS). For his service in these campaigns he was awarded the Iron Cross 2nd Class on 15 December 1940. Units under his command on the Eastern Front in 1941 were responsible for the deaths of over 17,000 civilians during the Pripyat swamps punitive operation in the Byelorussian SSR. As commander of the 8th SS Cavalry Division Florian Geyer in 1943, he was involved in operations against partisans as well as defensive operations against the Red Army, for which he was awarded the Close Combat Clasp in bronze.
After being seriously wounded in September 1943, Fegelein was reassigned by Heinrich Himmler to Hitler's headquarters staff as his liaison officer and representative of the SS. Fegelein was present at the failed attempt on Hitler's life on 20 July 1944. He was on duty at Hitler's Führerbunker in Berlin in the closing months of the war, and was shot for desertion on 28 April 1945, two days before Hitler's suicide. Fegelein was an opportunist who ingratiated himself with Himmler, who granted him the best assignments and rapid promotions. Historians William L. Shirer and Ian Kershaw characterize him as cynical and disreputable. Albert Speer called him "one of the most disgusting people in Hitler's circle".