Fieseler was born in Glesch. 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 FAA 243, then with FAA 41. In 1917, he qualified as a fighter pilot and was posted on 12 July to the Macedonian front, initially flying a Roland D.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.