After returning to Germany in 1912, Creutzfeldt worked at the Neurological Institute in Frankfurt am Main, at the psychiatric-neurological clinics in Breslau, Kiel and Berlin, and at the "Deutsche Forschungsanstalt für Psychiatrie" in Munich. During the First World War, Creutzfeldt was deployed as a reserve medical officer and survived the sinking of the auxiliary cruiser SMS Greif, on which he was embarked. After being captured on February 29, 1916, he was repatriated as a doctor in May of that year and served in the Imperial Navy until the end of the war in 1918.