William was the second child and first son of Hermann, Prince of Wied (1814–1864), son of Johann August Karl, Prince of Wied and Princess Sophie Auguste of Solms-Braunfels, and his wife, Princess Marie of Nassau (1825–1902), daughter of William, Duke of Nassau and his first wife Princess Louise of Saxe-Hildburghausen. Through his mother he was descendant of William IV stadtholder of the Netherlands and George II of Great Britain.
