In 1895, he became a member of the municipal parliament in Charlottenburg, Berlin. Only in 1906 did Preuß become a full professor, at the "Berliner Handelshochschule" newly founded by local merchants. He taught there until 1918, when he also was "Rektor". His main focus was on constitutional law and on autonomous municipal administration ("kommunale Selbstverwaltung"). In 1906, the first volume of "Die Entwicklung des deutschen Städtewesens" was published. From 1910-18 he was honorary city councillor for the FVP. In this capacity he contributed to the project that what would later become the Greater Berlin Act. In 1912, he unsuccessfully ran for a seat in the Reichstag. In his widely read publication "Das deutsche Volk und die Politik" of 1915 he forcefully argued for a transformation of the "Obrigkeitsstaat" into a "Volksstaat".