Background
He was the fifth but third surviving son of Wilhelm, Duke of Saxe-Weimar and Eleonore Dorothea of Anhalt-Dessau. After the death of his father (1662), his older brother Johann Ernst II inherited Weimar, and his second brother Adolf Wilhelm received Eisenach. His fifth and only surviving son, Wilhelm August, was born eight days after his father"s death and became duke from the moment of his birth.
Johann George became the regent of the duchy and also the guardian of the new duke.