As per the wishes of his cousin King Christian VIII, Prince Johann first enlisted in the Prussian military in 1842 and upon his graduation was appointed second lieutenant of the 27th Prussian Infantry Regiment in Magdeburg. He studied at the University of Bonn before joining the Dragoon Guards Regiments in Berlin. He participated in the German revolutions of 1848–1849 and the First Schleswig War against Denmark; this conflict of loyalties between Prussia and Denmark prompted him to request exemption from service.[3] He went on to serve in various departments and was promoted to Rittmeister in 1854. The following year he was appointed Major à la suite and went to Denmark, settling down in Copenhagen.[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.[4]
Johann died unmarried in 1911, outliving the rest of his siblings. He was interred at Roskilde Cathedral.
^Driault, Edouard; Lhéritier, Michel (1926). Histoire diplomatique de la Grèce de 1821 à nos jours (in French). Paris. p. 235.{{cite book}}: CS1 maint: location missing publisher (link)
^ abcdefghijBille-Hansen, A. C.; Holck, Harald, eds. (1911) [1st pub.:1801]. Statshaandbog for Kongeriget Danmark for Aaret 1911 [State Manual of the Kingdom of Denmark for the Year 1911] (PDF). Kongelig Dansk Hof- og Statskalender (in Danish). Copenhagen: J.H. Schultz A.-S. Universitetsbogtrykkeri. p. 3. Retrieved 3 July 2020 – via da:DIS Danmark.
^Hof- und Staats-Handbuch des Herzogtum Anhalt (1867) "Herzoglicher Haus-orden Albrecht des Bären" p. 16
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