He was the son of Hermann Theodor Dietz, a restaurateur in Bromberg at 12 Schloss Straße (today's Grodzka Street).

While he gave up his tiring activity at the clinic for railwaymen, he moved his private practice downtown at 88-90 Gdańska Street[4] and set up as well a small sanatorium in the suburban village of Rynkowo (now a district of Bydgoszcz).

Though the signature of the Treaty of Versailles endorsed the return of Bydgoszcz to the reborn Polish nation, Hermann Dietz decided to stay in the city.

On January 19, 1920, he sided with Hugo Wolff, the German mayor, to take part in the ceremony handing over the municipal authority to Jan Maciaszek, the new city magistrate appointed by the Polish government.

In this situation, the new city council, afraid of riots, took the decision to preventively isolate some of the most active German leaders.

During the interwar period, he also gained a great respect among Poles, distinguishing himself by a favorable attitude towards them, whenever the tensions arose, with conflicts and disputes between both nationalities being a daily business.

Although he was regularly vocal in his critics upon Nazism, he was nevertheless received in 1941 by the German city authorities for a ceremony celebrating his 80th birthday.

[11] During World War I, Hermann Dietz together with Elimar Schendell and other doctors organized nursing courses[12] at the "Auguste-Viktoria-Heim" (today's Kuyavia Pomerania Cultural Centre at 6 Kościelecki Square), where young infants were coming not only from Bromberg city but also from other regions.