Faced with large debts accumulated during the reign of his father Joachim II, he instituted a grain tax which drove part of the peasantry into dependence on the exempt nobility. Though a staunch Lutheran opposed to the rise of Calvinism, he permitted the admission of Calvinist refugees from the wars in the Spanish Netherlands and France. He was succceeded by his son Joachim Friedrich.
Upon the death of Albert of Prussia his father Joachim II Hector had become co-inheritor of Prussia. Joachim II died in 1571 and Johann Georg received the margraviate Brandenburg and the duchy of Prussia.