Commercial law
After leaving Japan, Mosse settled in Königsberg to be a state supreme court judge. He was awarded an honorary doctorate from the University of Königsberg in 1903 and, during the following year, became honorary professor there for Civil procedural law and Commercial law. After his retirement in 1907, he returned to Berlin where he served on the City Council and advised the Berlin municipal administration on various legal matters. He took part in public affairs of the Jewish community.