In 1935, the German newspaper Frankfurter Volksblatt published Grönhagen's article on Kalevala folklore.

Reichsführer-SS Heinrich Himmler showed interest on his thoughts and asked Grönhagen to work for Ahnenerbe.

He also worked shortly as one of the institute's directors but was dismissed by professor Walter Wüst who considered Grönhagen incompetent.

[2] During World War II, the Nazi-minded Grönhagen worked for Finland's propaganda department and served as its military attaché in Berlin.

He first lived in Crete and later in Athens serving as the Master of the Christian Order Ordo Sancti Constantini Magni.