The duchies were later merged into Saxe-Gotha-Altenburg. He was the ninth but sixth surviving son of Johann II, Duke of Saxe-Weimar, and Dorothea Maria of Anhalt. His mother was a granddaughter of Christoph, Duke of Württemberg, and great-granddaughter of Ulrich, Duke of Württemberg
Like his father, Louis was not a gifted economist and only his good relationship with Empress Maria Theresa and her intervention at the Imperial Court Council kept the Landgraviate from bankruptcy. However, his caring for his country is documented by the establishment of a textile house in 1742 and a state orphanage in the 1746.