Following the joint German-Soviet invasion of Poland, which started World War II in September 1939, the town was occupied by Germany until 1945 and local Poles were subjected to extensive genocidal policies. On October 20, 1939, the German "Einsatzgruppe VI" carried out a public execution of 15 Poles in the town as part of the "Intelligenzaktion". Among the victims were teachers, doctors, merchants and craftsmen from Mosina and nearby villages. It was one of many massacres of Poles committed that day by Germany across the region in attempt to pacify and terrorize the Polish population. Poles from Mosina were also murdered by the German police and "Selbstschutz" in the nearby town of Murowana Goślina in September and November 1939. In 1939–1941, the occupiers carried out expulsions of Poles, whose houses, shops and workshops were then handed over to German colonists as part of the "Lebensraum" policy. In 1943, many Polish children were kidnapped and imprisoned by the Germans in a camp for Polish children in Łódź, which was nicknamed "little Auschwitz" due to its conditions. Many children died in the camp.