The son of a Protestant minister, he joined the Imperial German Navy in 1899 after leaving secondary school. Promoted to lieutenant commander by 1917, Ehrhardt returned to Wilhelmshaven after the First World War. At the end of January 1919, he was involved in the suppression of the soviet republic as commander of the government troops. Under his command, the 2nd Marine Brigade, soon referred to as the Brigade Ehrhardt, became a formidable unit against communist uprisings. In April/May 1919, these nationalist and anti-democratic troops took part in the defeat of the soviet republic in Munich.
Ehrhardt was one of those responsible for the Kapp Putsch in Berlin in 1920. After its collapse, Ehrhardt escaped to Munich. There, he and his staff founded the terrorist underground organization "Organization Consul" in fall 1920 under the protection of Munich's police president Ernst Pöhner; its goal was violent destruction of the republic. It was responsible for the assassination of the Reich Ministers Matthias Erzberger (1921) and Walther Rathenau (1922), and possibly also for the murder of the Bavarian Member of Parliament Karl Gareis (1921).
Ehrhardt also maintained contact with Ernst Röhm and the early Hitler movement. In 1921, he assigned instructors from his staff for the establishment of the SA. Ehrhardt was arrested in November 1922 for his involvement in the Kapp Putsch, but managed to escape in July 1923. He returned to Munich in September 1923.
During the Hitler putsch, Ehrhardt sided with Gustav von Kahrs, who distanced himself from Adolf Hitler. He was therefore considered a traitor by the National Socialists. From 1924 to 1926, Hermann Ehrhardt went into hiding in Austria and lost his political influence. After the Nazis' seizure of power, he demonstratively sided with the new authority, but was unable to shed the stigma of his former adversary. In the context of the "Night of the Long Knives" in 1934, Ehrhardt was threatened and fled abroad. Starting in 1936, he lived in seclusion in Austria and no longer made any military or political appearances in the Reich. He became an Austrian citizen in 1948.
