After joining the Württemberg judiciary in 1849 and assisting at various court locations, he became Senior Justice Assessor (German: Obertribunalrat) in 1854 and Public Prosecutor in Ellwangen in 1857.

[2] From 1861 to 1900, Mittnacht was a Deputy for the Oberamt Mergentheim in the lower house (German: Abgeordnetenhaus) of the Württemberg.

He was much more committed than Varnbüler in the negotiations on the accession of the Kingdom of Württemberg to the newly emerging German Reich.

[5] Mittnacht, who was sympathetic to Greater Germany at the beginning of his political career, had been loyal to the German Reich since 1871.

In the spirit of the royal couple King Charles I and Queen Olga,[6] however, he paid attention to maintaining the Federal structure of the Empire, in particular by sticking to important reserve rights for the Kingdom of Württemberg (e.g. Württemberg's own foreign ministry,[3] its own legations abroad,[3] its own Army,[7] its own taxes as well as its own postal and railway system).

In 1900, Mittnacht resigned from all his government offices and the State Parliament due to his advanced age and illness,[10] choosing to spend his retirement in Friedrichshafen on Lake Constance.