Duncker was born in Hamburg as a merchant's son. He studied music at the Leipzig Conservatory, then history, economics and philosophy at the University of Leipzig. He joined the Social Democratic Party of Germany in 1893. In 1898 he married Käte Duncker who was then a teacher, but also became a socialist politician, journalist and feminist. The couple had three children: daughter Hedwig, and sons Karl and Wolfgang. In 1900, Duncker started teaching at the Leipzig workers' educational association. In 1903 he completed his Ph.D. under supervision of Karl Bücher and Karl Lamprecht. In the same year, Duncker became a journalist at the SPD-affiliated Leipziger Volkszeitung. In 1904, he founded a "workers' secretariat" in Leipzig, and in the following year in Dresden. In 1907, the family moved to Stuttgart from where Hermann Duncker toured the country as an itinerant teacher for workers' education. During the First World War, Hermann and Käte Duncker were protagonists of the SPD's leftist, internationalist and pacifist wing. Together with Karl Liebknecht, Rosa Luxemburg and Clara Zetkin they were among the founders of the Spartacus League that became the Communist Party of Germany during the German Revolution of 1918–1919. Both Käte and Hermann Duncker were elected to the KPD's first central committee. He resumed his lecture tours and directed KPD schools on the regional and national level. In 1925 he co-founded the Berlin Marxist Workers' School. In the KPD central committee he was responsible for education and instruction. He represented the relatively moderate "Middle Group" within the party that aimed for a united front with the Social Democrats. Therefore, he was sidelined from the radicalising party leadership after 1929. After the Nazis'seizure of power, Duncker, like most communist leaders, was taken into "protective custody" in February 1933, but he was released in November of the same year. In 1936 he emigrated to Denmark, then to England and France. Duncker was distraught over the persecution of his son Wolfgang and his comrade and friend Nikolai Bukharin during Stalin's Great Purge in the Soviet Union. Duncker also fell out with the KPD's Moscow leadership over the Molotov–Ribbentrop Pact between the Soviet Union and Nazi Germany, which he strongly opposed. Käte Duncker had fled to the United States, living with their son Karl who suffered from depression and committed suicide in 1940. When the German Wehrmachtinvaded Francein the Summer of 1940, Duncker fled from Paris to Vichy France's so-called zone libre. His wife organised a visa for Duncker's entry to the United States in late-1940. During a stopover in Casablanca, he was temporarily detained, and only arrived in New York in September 1941. In the US, Duncker joined the Council for a Democratic Germany in 1944. After the end of the war, Käte and Hermann Duncker returned to Germany in May 1947. He joined the ruling Socialist Unity Party of Germany. In the same year he was appointed professor at the University of Rostock, teaching history of social movements, and dean of the faculty of social sciences. In the German Democratic Republic, Duncker became the rector of the Free German Trade Union Federation academy in Bernau bei Berlin in 1949. Almost blind by that time, he held that position until his death. From 1955 to 1960 he was also a member of the East German trade union federation's executive board. Duncker was awarded the Order of Karl Marx in 1953, an honorary doctorate of the University of Leipzig in 1954 and the highest class of the Patriotic Order of Merit in 1955. He was buried near the Memorial to the Socialists at the Berlin Friedrichsfelde central cemitery.
