Of German descent,[1] he served in the Austro-Hungarian Army in World War I, and fought in the Eastern front, stationed at Sighetu Marmației, where he received a decoration for bravery and was promoted to a major.

While in Tomsk, Münnich joined the Communist Party of the Soviet Union, then served as a commander of an international POW unit fighting for the Bolsheviks.

After the dissolution of the HSR, he joined Béla Kun's faction and participated in the March Action in the Weimar Republic, which led to his arrest and deportation back to Hungary.

Münnich resided in the Soviet Union from 1922-1936, and joined the board of the Hammer and Sickle magazine (Sarló és Kalapács (folyóirat) [hu]), also serving as the editor from 1931-1933.

He joined János Kádár in the CPSU Presidium meetings in Moscow in 1956 and was a key member of the Hungarian Socialist Workers' Party.