Johann Georg received an income from the new duchy of Saxe-Eisenach and took his residence in the small town of Marksuhl.

Wilhelm August died in 1671 at only two years of age, and Johann Georg inherited the duchy.

The next year (1672) the death without heirs of the Duke Frederick Wilhelm III of Saxe-Altenburg forced a new treaty of division of the family lands between Johann Georg, his surviving brothers and his cousin.

Johann Georg was confirmed in his possession of Eisenach and took some towns, and his younger brother, Bernhard, inherited Jena.

In the spring of 1672, when Johann Georg officially moved his residence from Marksuhl to Eisenach, the occasion was marked by a special Easter church service with music composed by Ambrosius Bach.