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).
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:
Nevus Sebaceous of Jadassohn: yellow-orange flat plaque, occurring most commonly on the scalp (60%) or face (30%), usually present at birth, malignant potential (estimated 1%, most commonly basal cell carcinoma), surgically excised prior to hormone-triggered growth during puberty
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).