His elder brother, Frederick IV, succeeded their father as ruler of the duchy, Christian August being given the small fiefdom of Eutin in 1695, whereupon he took the title Duke of Holstein-Eutin.

[1] Additionally, he was appointed coadjutor of Lübeck, a Lutheran Imperial state within the Holy Roman Empire, in 1701, and his family managed to have him elected as the bishop on 26 April 1706.

[1] His eldest brother died in 1702, leaving only an underage son, Charles Frederick, Duke of Holstein-Gottorp, as his heir.

From 1702 to 1708 Christian August was co-regent with his widowed sister-in-law, Hedvig Sophia of Sweden, for Charles Frederick, having been first installed as administrator under her authority.

[1] Upon her death in 1708, Christian August became sole regent of Holstein-Gottorp, which duchy was severely ravaged by the violence of the Great Northern War.