He studied at the University of Bonn before joining the Dragoon Guards Regiments in Berlin.

The following year he was appointed Major à la suite and went to Denmark, settling down in Copenhagen.

[2] When the Second Schleswig War broke out in 1864, Johann resigned from the Prussian army, and on 29 February his brother — now King Christian IX of Denmark — appointed him Lieutenant Colonel à la suite in the Danish Army.

He went on to represent Denmark on diplomatic visits abroad, and was present in London during the baptism of the Prince of Wales' son, Johann's great-nephew, the future King George V of the United Kingdom.

[2] From March to November 1867 he served as regent for his nephew, King George I of Greece during the Cretan uprising, when the latter was away on a tour of Europe in search of a bride;[2] he soon grew popular among the people.