After the death of his father (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.
Johann Ernst (b. Weimar, 25 December1696 - d. Frankfurt, 1 August1715), a composer who studied with Bach and of whose concertos Bach made transcriptions .
After Johann Ernst's death, his successor and eldest son, Ernst August, was nominally given his father's power, but the real power was retained by his uncle, Wilhelm Ernst, until his death in 1728, when Ernst August, became the sole reigning duke of Saxe-Weimar.