[1] John was born on 25 March 1545 in Haderslev in the Duchy of Schleswig as the fourth child and third son of King Christian III of Denmark and Norway and his wife, Dorothea of Saxe-Lauenburg.

Where the eldest, Frederick II, had long ago been appointed successor to the thrones of Denmark and Norway, all three brothers were in principle equally entitled to the father's share of the duchies of Schleswig and Holstein.

To avoid unfortunate divisions of the royal part of the duchies, attempts were made to find suitable positions for the younger brothers elsewhere.

However, the plan to secure John the post of the prince-archbishop's successor in the Archdiocese of Bremen failed due to strong competition from North German princes.

Other options abroad were thwarted because Denmark needed the support of the North German princes against Sweden in the Nordic Seven Years' War, and the king could not act as a competitor in acquiring lucrative offices and benefices for his brother.