 When only three years old (1678) he succeeded his father as Duke of Saxe-Jena.In accordance with the testamentary instructions of his father, his uncle, Duke Johann Ernst II of Saxe-Weimar assumed his guardianship and the regency of the duchy; when he died in 1683, another uncle, Duke Johann Georg I of Saxe-Eisenach, assumed the regency.Three years later (1686), the new regent also died, and his cousin (son of the late Duke Johann Ernst II) and brother-in-law (he was married to his eldest and only surviving sister, Charlotte Marie) Duke Wilhelm Ernst of Saxe-Weimar was appointed to the regency.