Little is known of his early years, but in 1180 Hermann joined a coalition against Henry the Lion, Duke of Saxony, and with his brother, Landgrave Ludwig III of Thuringia, suffered a short imprisonment after his defeat at Weissensee by Henry. About this time he received from his brother Louis the Saxon palatinate, over which he strengthened his authority by marrying Sophia, sister of Adalbert, count of Sommerschenburg, a former count palatine.
Louis II died in 1190. Emperor Henry VI attempted to seize Thuringia as a vacant fief of the Holy Roman Empire, but Hermann frustrated the plan and established himself as the landgrave. Having joined a league against the emperor, he was accused, probably wrongly, of an attempt to murder him. Henry VI was not only successful in detaching Hermann from the hostile combination, but gained his support for the scheme to unite Sicily with the Empire.
Hermann went on crusade in 1197. When Henry VI died in 1198, Hermann's support was purchased by the late emperor's brother Duke Philip of Swabia, but as soon as Philip's cause appeared to be weakening he transferred his allegiance to Otto of Brunswick, the later Emperor Otto IV. Philip accordingly invaded Thuringia in 1204 and compelled Hermann to come to terms by which he surrendered the lands he had obtained in 1198. After the death of Philip and the recognition of Otto, Hermann was among the princes who invited Frederick of Hohenstaufen, afterwards Emperor Frederick II, to come to Germany and assume the crown. In consequence of this step the Saxons attacked Thuringia, but the landgrave was saved by Frederick's arrival in Germany in 1212.
After the death of his first wife in 1195, Hermann married Sophia, daughter of Otto of Wittelsbach. By her he had four sons, two of whom, Ludwig IV of Thuringia and Heinrich Raspe, succeeded their father in turn as landgrave. Hermann died at Gotha in 1217 and was buried at Reinhardsbrunn.
Hermann was fond of the society of men of letters, and Walther von der Vogelweide and other Minnesingers were welcomed to his castle, the Wartburg. In this connection he figures in Richard Wagner's Tannhäuser.