He was born in London the son of Albrecht Wolfgang, Count of Schaumburg-Lippe and of his first wife Countess Margarete Gertrud of Oeynhausen (1701–1726), an alleged bastard daughter of George I of Great Britain and his mistress Ehrengard Melusine von der Schulenburg.In 1762, at the request of the Marquis of Pombal (the Portuguese Secretary of State) he led, as Generalissimus, the allied troops in Portugal against the Spanish invasion.William conducted a brilliant defensive campaign of marches and counter-marches, so that the enemy, although it had a three-to-one superiority in numbers, always encountered defenders in a good position and never dared to risk an all-out attack.Years after, before her marriage, Olimpia was recognized by her father's cousin, Philip II Ernst, and her name was changed to Olímpia Patronellia Ernestina de Schaumburg-Lippe.He was married on 12 November 1765 at Stadthagen to Countess Marie Barbara Eleonore of Lippe-Biesterfeld (1744–1776), daughter of Frederick Charles Augustus, Count of Lippe.