Wilhelm August von der Osten was born in Copenhagen, the son of lord chamberlain Peter Christoph van der Osten (1659–1730) and Louise Benedicte von Reichow (1670–1755).
In 1743, he left Sorø to assume a position as director of the financial administration in Copenhagen and was also appointed as member of the treasury (rentekammeret).
He vigorously advocated a limitation of state expenditure, leading to the establishment of a commission consisting of J. L. Holstein, C. A. Berckentin, J. S. Schulin and himself that was to propose budget cuts.
His second wife was Friederike Anna Sophie von Massow (ca.
Van der Osten constructed one of the first town mansions in the new Frederiksstaden district in Copenhagen.