In 1739 William inherited the estates formerly owned by the Nassau-Dillenburg branch of his family, and in 1743 he inherited those formerly owned by the Nassau-Siegen branch of his family.
In 1740, the War of the Austrian Succession broke out. The conflict pitted Austria against France over the issue of whether Maria Theresa had the right to inherit her father Emperor Charles VI's crown. The Dutch Republic sided with Austria in 1747 in order to maintain a buffer zone between itself and France, whereupon French troops invaded the Austrian Netherlands. In a few weeks, Louis XV's troops conquered most of the towns in the Austrian Netherlands where the Dutch had stationed troops under the Barrier Treaty, as well as the most important towns in Zeelandic Flanders. The Dutch Republic was at the time weakened by internal division. The Dutch decided that their country needed a single strong executive, and turned to the House of Orange. William and his family moved from Leeuwarden to The Hague. On 4 May 1747, the States General of the Netherlands named William General Stadtholder of all seven of the United Provinces of the Netherlands, and made the position hereditary for the first time. William first met Duke Louis Ernest of Brunswick-Lüneburg in 1747, and two years later appointed him field marshal of the Dutch States Army, which later led to Louis Ernest serving as one of the regents for William's heir.
William IV was considered an attractive, educated, and accomplished prince in his prime. Although he had little experience in state affairs, William was at first popular with the people. He stopped the practice of indirect taxation by which independent contractors managed to make large sums for themselves. Nevertheless, he was also a Director-General of the Dutch East India Company, and his alliance with the business class deepened while the disparity between rich and poor grew.