Fasch was born in the town of Buttelstedt, 11 km north of Weimar, the eldest child of schoolmaster Friedrich Georg Fasch and his wife Sophie Wegerig, from Leißling near Weißenfels. After his father's death in 1700, Fasch lived with his mother's brother, the clergyman Gottfried Wegerig in Göthewitz, and it was presumably in this way that he made the acquaintance of the Opera composer Reinhard Keiser.