In 1572 he served as a captain with Spanish forces and saw duty in The Netherlands under the great general Alexander Farnese, Duke of Parma and Piacenza.

Ruprecht von Eggenberg benefited greatly from this experience and was ultimately appointed by Charles II, Archduke of Austria to the arch-ducal council and to captain of the guard at the main castle in Graz.

Although outnumbered, Eggenberg vanquished the Ottoman forces under the command of Hasan Pasha Predojević at the Battle of Sisak on 22 June 1593.

In the end, due to his distinguished service, Ruprecht von Eggenberg was elevated to the rank of Freiherr in 1598 by Rudolf II.

Furthermore, Ruprecht von Eggenberg had mandated in his testament that only male, Catholic members of the family who had attained the rank of general or higher may be buried in his mausoleum.