He inherited the East Frisia after the unexpected death of his brother Rudolf Christian on 17 April 1628.

His brother Rudolf Christian died unexpectedly, from a stab in his left eye during an argument with a lieutenant in the army of general Matthias Gallas quartered at Berum Castle.

In the face of the foreign troops quartered in East Frisia during the Thirty Years' War, he was rather passive and let his chancellors Wiarda and Bobart manage the country.

He even went so far as to build a Lustschloss, the Julianenburg in Sandhorst, for his wife Landgravine Juliana, in the middle of the war, while the population of East Frisia was suffering bitterly.

He leased out parcels of bog around Timmel in 1633; this is regarded as the starting point of the fen cultivation, which led to the creation of the Großefehn.