In 1527, Jagow was elected as bishop of Brandenburg, as successor of Hieronymus Schulz and Dietrich V of Hardenberg, two determined opponents of Martin Luther.

Although convinced of the need for ecclesiastical reform, he contented himself during the lifetime of anti-reform Elector Joachim I with the redress of abuses and the improvement of clergy.

As the most influential advisor of his successor, Joachim II, von Jagow convinced Joachim II to convert to Lutheranism and presented the communion under both kinds to him on 1 November 1539, and to the magistrate of Berlin on 2 November.

Around 1535, he had a bishop's cap constructed on top of the Bergfried of his residence Ziesar Castle, as a sign of its being used by the church.

Magnussen presented the bishop as a bearded, elderly man with a decorative raised collar.