Sigismund succeeded his elder brother, Frederick, in 1552 as prince-archbishop of Magdeburg and diocesan administrator of the Prince-Bishopric of Halberstadt.
Until 1557, when he came of age, Count John George of Mansfeld was installed by the cathedral chapter to run the prince-archbishopric.
On 21 January 1553 the young prince-archbishop, who had meanwhile been confirmed by Pope Julius III although he was Lutheran, was rendered homage by the nobility in Halle upon Saale.
Three years later, Emperor Ferdinand I granted the prince-archbishop the privilegium de non appellando.
His death at age 28 prevented him being a serious candidate for the Polish throne and hindered the introduction of Reformation to the whole archdiocese.