Creutzfeldt was born into a medical family, on June 2, 1885, at Harburg an der Elbe, Germany, which was incorporated into Hamburg in 1937.

After qualification he sought adventure as a ship's surgeon, voyaging the Pacific Ocean, taking the opportunity to study local crafts, linguistics, and tropical plants.

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 German Navy until the end of the war in 1918.