 He served as administrator of the Archbishopric of Magdeburg from 1566 to 1598, then succeeded his father as Elector of Brandenburg in 1598.Joachim Frederick was succeeded at his death by his son John Sigismund.Joachim Frederick's first marriage on 7 March 1570 was to Catherine of Brandenburg-Küstrin, daughter of John, Margrave of Brandenburg-Küstrin, and Catherine of Brunswick-Wolfenbüttel.