Their marriage resulted in the potential creation of Brandenburg-Prussia, which became a reality after Poland's leader appointed John Sigismund in charge of Prussia in regency and, shortly thereafter, Albert Frederick died without an able, direct male heir.

John Sigismund was born in Halle an der Saale to Joachim III Frederick, Elector of Brandenburg, and his first wife Catherine of Brandenburg-Küstrin.

John Sigismund gave the Reichshof Castrop to his teacher and educator Carl Friedrich von Bordelius.

John Sigismund's most significant action was his conversion from Lutheranism to Calvinism, after he had earlier equalized the rights of Catholics and Protestants in the Duchy of Prussia under pressure from the King of Poland.

After the Elector and his Calvinist court officials drew up plans for mass conversion of the population to the new faith in February 1614, as provided for by the rule of Cuius regio, eius religio within the Holy Roman Empire, there were serious protests, with his wife backing the Lutherans.