Duke of Saxe-Weimar as a son of Duke Johann II of Saxe-Weimar and his wife Dorothea Maria of Anhalt. His brothers were the Dukes Johann Ernst I “the Younger” of Saxe-Weimar, Friedrich of Saxe-Weimar, Wilhelm IV of Saxe-Weimar, Albrecht IV of Saxe-Eisenach, Ernst I of Saxe-Gotha and Bernhard of Saxe-Weimar.
Johann Friedrich, like his brother Albrecht, was made a member of the Fruitbearing Society by Prince Ludwig I of Anhalt-Köthen, before the start of their Grand Tour. Ludwig gave Johann Friedrich the nickname ‘der Entzündete’ (“the Inflamed”) and the motto ‘verderbet und erhält’ (“spoil and receive”).
In 1622, Johann Friedrich and his brother Bernhard fought in the Battle of Wimpfen on the side of Baden. Three years later, his brother Johann Ernst the Younger promoted him to Colonel. Later that year, a power struggle between the brothers escalated for political reasons. It ended when Johann Friedrich was arrested. He was later released. However, in 1627 he attempted to join the army of Tilly. He was caught, and again imprisoned by his brothers.
Johann Friedrich was very interested in Alchemy all his life. On 16 October 1628, while still in prison, he confessed, in writing, to a pact with the devil. The next day, he was found dead in his cell. Speculations ranged from suicide to murder; neither was ever conclusively proven. A witch trial against him was never started.
Link to Wikipedia biography