He was the third child and first surviving son of Johann Adolf I, Duke of Saxe-Weissenfels, by his first wife, Johanna Magdalena of Saxe-Altenburg. Johann Georg succeeded his father in the duchy of Saxe-Weissenfels upon his death on 24 May 1697.
Because he was still a minor, the Elector Frederick August I of Saxony briefly assumed a regency. Like his both predecessors, Johann Georg was interested in developing a flotilla, but he was also a great patron of the arts and sciences. Under his rule Weissenfels became the leading economical and cultural center in central Germany along with Dresden.
To maintain order during civic celebrations, Johann George created the establishment of Citizen Companies (Bürgerkompanien), in whose service male inhabitants were conscripted.
The statutes of the order, which the duke wrote both in German and in French, required an irreproachable life and noble birth for admittance. During the Great Northern War, Weissenfels was occupied by Swedish troops from 1706 to 1707.
