A native of Hamburg, Stahmer fought during World War I and earned both classes of Iron Cross.

In 1936, Stahmer took part in the negotiations for the Anti Comintern Pact between the German and the Japanese governments.

Throughout 1940, he worked for a German-Japanese alliance, and on 13 August 1940, he was able to notify the Japanese embassy in Berlin about the decision to conclude such a treaty.

In October 1941, Stahmer was appointed as German ambassador to the collaborationist Chinese reorganised national government under Wang Jingwei, established in Nanjing by the Japanese occupation,[3] and remained in that position until late 1942.

[5] After the surrender of the German government, the Japanese government broke off diplomatic relations with the German Reich on 15 May 1945, and Stahmer was interned and kept under arrest in the Fujiya Hotel in Hakone near Tokyo until the Japanese surrender in August 1945.