Hartmann was a member of the Swabian noble von Dillingen family, who held territory in the Upper Danube area and the office of Vogt over the city of Ulm.

In 1241, Hartmann and his father, the Count, donated to the Community of Ladies in Dillingen a house near the parish church with one lot of land, a cabbage patch and a meadow.

In 1248, Siboto of Seefeld was deposed as Bishop of Augsburg by Pope Innocent IV and Hartmann was appointed as his successor.

Augsburg, like other large cities throughout the greater part of Germany, attained enormous wealth, owing to the industrial and commercial activity of the citizens.

In 1251 Bishop Hartmann granted the Dominican sisters of Augsburg land in the parish of S. Moritz, where the nuns built a new cloister and church.

In a dispute with Count Louis III of Oettingen about the office of Vogt over Neresheim Abbey, an arbitration board chaired by Albertus Magnus ruled against him.