In June 1919 the First Ukrainian Soviet Division destroyed Ukrainian troops and reinstated the Soviet regime in Mlyniv. After that Mlyniv was governed by local kombed. During the Polish–Soviet War, Mlyniv was changing hands between various armies. In July 1920 for short period of time the Soviet regime once again was secured on the efforts of the 14th Soviet Cavalry Division (commander , part of the First Cavalry Army, Konnarmia). By the 1921 treaty of Riga, Mlyniv became part of the Second Polish Republic (Wołyń Voivodeship). In 1921 Mlyniv was accounted for some 1263 people. As part of Poland, in Mlyniv Polish language became enforced (revived) as the official. In 1932 there was built a state seven-grade school. There also was opened a local pharmacy. In 1930 population of Mlyniv declined in comparison with 1921. In February 1926 in Mlyniv was created a local committee of Communist Party of Western Ukraine and since 1935 its youth wing Komsomol. To fight increase of communist influence a local starosta requested to increase number of local police force. In September 1934 some demonstrations organized by local communists resulted in arrest of 23 people. Another communist actions took place in February 1938 in relations to free Belarusian revolutionary Siarhei Prytytski. The action called to help Republican Spain (Spanish Civil War), fight fascism, democratization of Poland and liberation of West Ukraine. In Mlyniv existed Catholic parish within Dubno county. Soon after the start of World War II and next partition of Poland by Nazi Germany and Soviet Union the village as part of West Ukraine became again occupied (liberated) by the Red Army. Near Mlyniv was stationed for two years the 8th Mechanized Corps (commander Dmitry Ryabyshev). A number of prominent Polish families were sent to Siberia by the NKVD in 1940. In October 1939 in Mlyniv took place elections to the National Assembly of Western Ukraine. Following admission of Western Ukraine to the Soviet Union and unification with the Ukrainian SSR, Mlyniv became an administrative center of Mlyniv Raion. In Mlyniv was opened local club, library, maternity hospital, and small clinic. Mlyniv was overrun by the German army in Operation Barbarossa on 24 June 1941. The local school was transformed into headquarters of commandant and Gestapo. During the Nazi Germany occupation there were killed 1118 people. In repressions against local Soviet population participated Ukrainian nationalists.