In 1914, Müller joined the army and served in the infantry. He transferred to the German Air Force in 1916 and flew two-seaters until the end of 1917. In March 1918, after downing an R.E.8 for his third victory, Müller was shot down over no man's land but managed to make it back to his own lines. In May, Jasta 18 engaged the 103rd Aero Squadron and Müller shot down a SPAD for his fifth victory. His victim was probably Paul Baer, the first American ace of the United States Air Service. On 14 September 1918, Müller scored Jasta 18's 100th victory. That morning he encountered the 13th Aero and shot down three SPAD XIIIs in 15 minutes. Later the same day, he claimed a fourth SPAD from the same squadron.
At some point in time, Müller changed his surname to "Garrelt" as the name "Hans Müller" was rather common and he and a neighbor with the same name were forever getting their mail confused. During World War II, Müller served on the staff of Lutflotte III under Generalfeldmarschall Sperrle. In civilian live he became an engineer designing steam locomotives for Hanomag and Henschel before going freelance. Müller died of a brain tumor in 1964.