When Joachim I died on 11 July 1535, the Margraviate of Brandenburg-Küstrin was created for John; it consisted of the territories of the New March with Dramburg and Schivelbein, the Sternberg Land, Crossen an der Oder with Züllichau and Sommerfeld and the Loardship of Cottbus with Peitz, altogether about 12,500 square kilometers.

Joachim II, the elder son, inherited the title of Elector and the rest of the Margraviate of Brandenburg.

After he returned to his Margraviate of Brandenburg-Küstrin, John strengthened its defenses and started planning an anti-Habsburg alliance, which however did not come about.

A group of German princes led by Elector Maurice of Saxony, and supported by France, opposed the Interim of Augsburg.

After a dispute with Maurice in the hunting castle Annaburg in Lochau in 1551, John decided not to join this group.