Jahn studied natural sciences at the Humboldt University of Berlin, especially biology, until he received his doctorate in 1894 under Simon Schwendener.
Jahn then passed his teaching qualification and taught in Berlin-Charlottenburg at the Oberrealschule [de] until 1921.
[2] In November 1933 Jahn signed the Bekenntnis der deutschen Professoren zu Adolf Hitler.
[3] In 1906 he published his scientific description of Listerella paradoxa, which is a slime mould species from the class Myxogastria and the only member of its genus, as well as the family Listerelliidae.
[4][5] The fungal genus Jahnula, circumscribed by Wilhelm Kirschstein in 1936, is named in honor of Jahn and with it the order Jahnulales.