He was one of the most loyal vassals of King Alfonso III, who named him his mayordomo mayor[1] and compensated his efforts and services to the crown with many properties and tenencias.

In 878, he defeated the Muslim troops who had attacked Oporto and then Coimbra,[4] repopulating these cities, as well as neighboring Braga, Viseu, and Lamego, with people from Galicia after expelling the Moors.

[3][5] His holdings there would pass to his descendants and come to be called the County of Coimbra,[6] which was retaken again in 987 by Almanzor and it was not until 1064 that the city was permanently reconquered by the Christian armies of Ferdinand I of León.

In 895, Hermenegildo defeated and captured the Galician noble Witiza who had taken up arms against the king of Asturias, taking him in chains before the monarch who compensated the count with many of the rebel's estates and tenencias.

This marriage gave rise to one of the most prominent noble families in medieval Galicia and in the County of Portugal.