In 1920 he joined the newly formed Communist Party of Great Britain and founded and edited the magazine Labour Monthly from 1921 until his death. He was on the Executive Committee of the CPGB from 1923 until 1965, and was the party's chief theoretician for many years. He also played an important role for the Comintern by supervising the Communist Party of India for some years.
Dutt was unswervingly loyal to the Soviet Union and to Joseph Stalin. In 1939, when the CPGB leader Harry Pollitt supported Britain's entry into World War II, it was Dutt who promoted Stalin's line that the war should be opposed, and brought the party into line, forcing Pollitt's temporary resignation.
After Stalin's death Dutt remained a loyal Stalinist and he disagreed with the CPGB's criticisms of the Soviet invasion of Czechoslovakia in 1968. He opposed the CPGB's increasingly Eurocommunist line in the 1970s and retired from his party positions, although remaining a member until his death.