Following his return he was appointed the Interior Minister of Prussia on 8 December 1862. Following the growth in the administrative area of Prussian state in 1864 and 1866, Eulenburg instituted comprehensive administrative reform also in the old provinces. However he was met by opposition by the Catholics in the west and the liberals in the east. He accompanied King Wilhelm I of Prussia in the negotiations with the French ambassador Count Benedetti at Bad Ems prior to the Franco-Prussian War.