In December 1161 he was solemnly enthroned on the Duke's Chair in the Zollfeld plain, in the presence of Archbishop Eberhard of Salzburg and Patriarch Ulrich II of Aquileia.
With varying degrees of success he tried to consolidate his position in Carinthia by achieving the office of a Vogt protector over the Diocese of Gurk as well as over the Carinthian possessions of the Bishopric of Bamberg and the Patriarchate of Aquileia.
Upon the death of his uncle Margrave Engelbert III of Istria in 1173, Herman could not assume the margravial title, but inherited his personal possessions around the town of Sankt Veit which emerged as ducal residence.
About 1173, Herman married Agnes of Babenberg, daughter of Duke Henry II of Austria and widow of King Stephen III of Hungary.
[2] They had two known sons: Herman was the first Sponheim duke to be buried in St. Paul's Abbey in the Lavanttal, established by his great-grandfather Count Engelbert of Spanheim in 1091.