He first studied medicine in Erlangen and Heidelberg, where he was taught by Hermann von Helmholtz, and then continued his studies in Bonn and Bern. In Bern, he became assistant of the pathologist Edwin Klebs with whom he undertook his thesis. In 1870, he was conscripted as doctor for the military in Karlsruhe during the Franco-German war. During 1871-1875, Zahn was an assistant for Friedrich Daniel von Recklinghausen at Strasbourg, France, and at this time he described the microscopic structure of the thrombus in thrombosis. From 1876 until his death, he held the position of an associate professor of pathological anatomy in Geneva. He has published widely on the circulatory system including blood, thrombosis, embolism, arterial disease, and tumors. Zahn's name is associated with the Zahn infarct and the lines of Zahn.