He would serve in the army of the Grand Duchy of Saxony (Saxe-Weimar-Eisenach) and he was present when Wilhelm I was crowned as German emperor.

[1][2] Because his mother was a daughter of King William II of the Netherlands and his older uncles, except King William III, died childless, Karl August was second in line in the succession to the throne of the Netherlands from 1890 to 1894.

This was the reason for him learning to write and speak fluent Dutch, due to the possibility of Karl August becoming king if his cousin Queen Wilhelmina of the Netherlands died without issue.

He was also interested in the Grand Ducal Library and the newly built archive, where he often studied historical documents on the history of the house.

Karl August died at Cap Martin, France, six years before his father; because of this, his eldest son Wilhelm Ernst succeeded his grandfather as Grand Duke.