Christian Wilhelm was a son of Elector Joachim Frederick of Brandenburg (1546-1608) from his first marriage with Catherine (1549-1602), a daughter of Margrave John of Brandenburg-Küstrin (1513-1571). He was elected as Archbishop of Magdeburg in 1598. However, the city of Magdeburg did not recognize him as Prince-Bishop, because his election had not been confirmed by the emperor. When he married in 1614, he assumed the title of Lutheran administrator on. That same year, he also became coadjutor of Halberstadt. In 1624, he became administrator of Halberstadt. During the Thirty Years' War, he entered into an alliance with Denmark. In 1626, he led an army from Lower Saxony into the Battle of Dessau Bridge. After Wallenstein won this battle, he fled abroad. In 1629, he fled to the court of King Gustavus Adolphus of Sweden.