He was an assistant to Albert Neisser at the Allerheiligen Hospital in Breslau until 1892, the director of the university skin clinic in Bern (1896-1917), and later a professor of dermatology at Breslau University (1917-1932).
Jadassohn was among the first to take an immunological approach in research of dermatological disorders, and contributed to the understanding of the immunopathology of tuberculosis and trichophytosis. He was a pioneer in the field of allergology, and is credited for introducing patch testing for diagnosis of contact dermatitis. In 1901 he described a rare childhood dermatological disorder known as granulosis rubra nasi.
Two dermatological disorders that are named after him are: "Jadassohn's disease I" (a skin disorder originating at the elbow) and "Jadassohn's disease II" (a natal skin disorder affecting the face and scalp). Together with his assistants, Walter Dössekker (1868-1962), Max Tièche (1878-1938) and Felix Lewandowsky (1879-1921), he shares the following eponymous medical conditions:
- Jadassohn-Dössekker disease: Also known as atypical tuberous myxedema.
- Tièche-Jadassohn naevus: A hard, dark blue, benign melanocytic tumor.
- Jadassohn-Lewandowsky syndrome: An ectodermal dysplasia characterized by onychogryposis, hyperkeratosis, leukoplakia, hyperhidrosis and pachyonychia congenita.
Jadassohn published a revision of Edmund Lesser’s Lehrbuch der Haut- und Geschlechtskrankheiten (14th edition, 1927–30), and from 1927 published the multi-volume Handbuch für Haut- und Geschlechtskrankheiten. Other noted written works of his include:
- Die venerischen Krankheiten, (1901).
- Ueber eine eigenartige Erkrankung der Nasenhaut bei Kindern (Granulosis rubra nasi). Archiv für Dermatologie und Syphilis, Wien, 1901, 58: 145-158.
- Die Tuberkulose der Haut. In: Franz Mracek's Handbuch der Hautkrankheiten (1904).
- Allgemeine Ätiologie, Pathologie, Diagnose und Therapie der Gonorrhoe, (1910).
- Lepra, in Wilhelm Kolle and August Paul von Wassermann's Handbuch der pathogenen Microorganismen. second edition, Jena- 1913; third edition, volume 5,2- 1928.
- Dermatologie, (1938).
- Textbook of contact dermatitis by Richard J. G. Rycroft, et al