While at the university, his guardian appointed a companion and supervisor over the three princes, who later became Field Marshal Kaspar of Teutleben and the Preceptor Frederick Hortleder.

In 1613–1614, Johann and his brothers, with his guardians, took a tour of France, Great Britain and the Netherlands as part of their studies.

In 1615 Johann Ernst reached adulthood and took control of his duchy and the guardianship of his under-age younger brothers.

Later, he took a commission as a Danish lieutenant general of the cavalry and fought in the Thirty Years' War in Westphalia and Lower Saxony.

There Johann Ernst died, at the age of 32 years, in the Hungarian location of Sankt Martin, as a result of war wounds.