William V assumed the position of stadtholder and Captain-General of the Dutch States Army on his majority in 1766. However, he allowed the Duke of Brunswick to retain a large influence on the government with the secret "Acte van Consulentschap". On 4 October 1767 in Berlin, Prince William married Princess Wilhelmina of Prussia, the daughter of Augustus William of Prussia, niece of Frederick the Great and a cousin of George III. (He himself was George III's first cousin). He became an art collector and in 1774 his "Galerij Prins Willem V" was opened to the public.