He was the Governor of Kraków and Sandomierz from 1715 to 1724, and the member of the Senate of the Polish–Lithuanian Commonwealth, as the Castellan of Słońsk, from 1746 to 1758.
His father was a major general in the Imperial Army of the Holy Roman Empire, and the commander of the Petrovaradin Fortress.
From 1 January 1715 to 31 December 1724, he was the Governor of Kraków and Sandomierz, administrating the crown land.
In 1726, for his service to the country, the Seym of the Polish–Lithuanian Commonwealth granted him the indygenat, which was the recognition of his noble status.
There, he developed the local castle, and founded the construction of the Church of the Visitation of the Blessed Virgin Mary between 1742 and 1750.
[7] Wilhelm Mier was married to Katarzyna Barbara Geshaw, with whom he had three children; two sons and one daughter.