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

After his defeat in the Battle of White Mountain on 8 November 1620, the duke refused to submit to the Emperor without conditions.

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.