He was a son of Louis II, Count of Nassau-Weilburg and his wife Anna Maria of Hesse-Kassel (1567–1626).
His father, Louis II, had been able to unite all the territories of Walram branch of Nassau.
After his death in 1627, the territories were divided among four brothers, with William Louis, the oldest, acting as regent for the youngest two.
When Otto, the youngest died in 1632, and Ernest Casimir came of age, the three surviving brothers divided the territory anew.
[2] During this period, the brothers concluded the Treaty of Gotha of 16 July 1651, in which division of 1632 was confirmed and again adjusted.