After Ulrich's death, Hermann took over the custody of his nephew William and ruled as the de facto head of the family.
Under his rule, the House of Celje began expanding its influence from its power base in present-day Slovenia and southern Carinthia to Central Europe and the Balkans.
By the time of his death, Hermann I was the largest landowner in the territory of present-day Slovenia, where his possessions significantly outnumbered those of his Habsburg liege lords.
He was the second-born son of Frederick of Sanneck, first Count of Celje, and his wife Diemut from the noble Wallsee family.
This was in accordance with the family tradition which avoided splitting the inheritance among brothers, and instead opted for shared rule among them.