At 16 years of age, he resisted the rules set forward by the Allied Disarmament Commission for Germany, by taking part in exercises involving the camouflage of weapons.
[11] Later on, Mehlhorn became an expert on armaments and military policy and used to give lectures in meetings organized by youth sports societies such as Turnerschaft Mundenia.
[9] Mehlhorn attended university from January 1923 to December 1926 for a total of 8 trimesters, studying in Göttingen, Munich and Leipzig, eventually obtaining his Ph.D. in 1928.
His grade was very high and upon graduation he was the only one from his class not to become a civil servant, opting instead to join the ranks of a famous law office at Chemnitz.
He was deemed important enough to be hired along with Lothar Beutel, Hermann Behrends, and Wilhelm Albert in order to help with the formation of SD as an organization.
[13][14] In 1935, Mehlhorn was sent to the Berlin headquarters of the SD to assume a high-echelon position, probably due to legal advice he provided concerning the financial affairs of Heydrich's mother.
[10] Mehlhorn was the head of Department I in the Warthegau which was responsible for the General, Domestic, and Financial affairs of the Reichsstatthalter and also assisted in drafting many of the orders issued by Greiser.