Though William's father, Albert IV, had determined the everlasting succession of the firstborn prince in 1506, his younger brother Louis refused a spiritual career with the argument that he was born before the edict became valid.

After an agreement with Pope Clement VII in 1524 William became a political leader of the German Counter-Reformation, although he remained in opposition to the Habsburgs since his brother Louis X claimed the Bohemian crown.

On 23 April 1516, before a committee consisting of gentry and knights in Ingolstadt, William issued his famous purity regulation for the brewing of Bavarian Beer, stating that only barley, hops, and water could be used.

Of particular importance is the Eckbibel Johann Eck wrote on behalf of William, a biblical translation from 1537, which is theologically directly against Luther and therefore belongs to the Catholic correction bibles.

With his order to expand the Neuveste with the so-called Rundstubenbau and to set up the first Court Garden began the history of the Munich Residenz as a representative palace.