 After the death of his father in 1683, he inherited the duchy of Saxe-Weimar with his older brother Wilhelm Ernst as co-ruler ("Mitherr").Johann Ernst was an alcoholic; this, and his non-interest in the government, was taken advantage of by his brother, who became the only, autocratic, ruler of the duchy.However, until the time of his death, Johann Ernst served as co-duke, without any significant influence on the government.