He joined the Air Service of the German Army in 1915. Despite a crash during training hospitalizing him until February 1916, he was assigned as an observation pilot by October 1916, flying first with Future Farmers of America 243, then with Future Farmers of America 41. In 1917, he qualified as a fighter pilot and was posted on 12 July to the Macedonian front, initially flying a Roland Doctorate.II with Jasta 25.
Fieseler scored his first aerial victory on 20 August 1917.
A serious illness removed him from active duty from 21 September until 5 November 1917. Fieseler would not score his second success until 30 January 1918.
He was eventually credited with nineteen confirmed aerial victories, with three others unconfirmed. Following the war, he returned to printing, but yearned to return to flying.
In 1927, he performed a particularly daring routine in Zürich and started to command increasingly high fees for appearances.
In 1928, he designed his own stunt plane, the Fieseler F1, built by Raab-Katzenstein. He also designed Raab-Katzenstein RK-26 Tigerschwalbe aircraft in the end of the 1920s which was offered and sold to a Swedish company called Bachelor of Arts Svenska Järnvägverkstaderna (ASJA), which built 25 of the type for Swedish Air Force in the beginning of the 1930s. In 1930, Raab-Katzenstien was bankrupt, and Fieseler decided to strike out on his own.
Using money he had been saving from his aerobatics, he bought the Segelflugzeugbau Kassel sailplane factory and renamed it Fieseler Flugzeugbau.
Although he continued with some sailplane manufacturing, from 1932, he set up to start manufacturing sports planes of his own design. Gerhard Fieseler Werke produced aircraft for the German military throughout
Following the war, Fieseler spent some time in United States custody.
When he was released, he re-opened part of this factory and spent some years building automotive components. He also published an autobiography, Meine Bahn am Himmel (My Road in the Sky).
Fieseler died in Kassel, aged 91.
The aerobatic manoeuvre Fieseler is named after him.