n 1918, the Austro-Hungarian Monarchy collapsed and the Syrmia region first became a part of the newly formed State of Slovenes, Croats and Serbs, and then, on November 24, 1918, the assembly of Syrmia in Ruma decided that most of Syrmia (including Mitrovica) join to the Kingdom of Serbia. Subsequently, on December 1, 1918, Kingdom of Serbia united with the Kingdom of Montenegro and the State of Slovenes, Croats and Serbs to form the Kingdom of Serbs, Croats and Slovenes (renamed to Yugoslavia in 1929). Between 1918 and 1922, Sremska Mitrovica was part of the Syrmia County, between 1922 and 1929 part of the Syrmia Oblast, between 1929 and 1931 part of the Drina Banovina, and between 1931 and 1941 part of the Danube Banovina.
During World War II, the city was occupied by Axis troops and was attached to the Independent State of Croatia. During that time its name was changed to Hrvatska Mitrovica (meaning Croatian Mitrovica). One of the largest Nazi concentration/death camps in the Independent State of Croatia existed in Sremska Mitrovica and as many as 10,000 victims (Serbs, Jews, and antifascists) were killed here. The Serbian Jewish population was to be interned in a concentration camp built first in Jarak and then at Zasavica. However, both locations proved to be too flooded for construction. The Germans had to abandon these locations and use Sajmište, which resulted in the destruction of 83% of Serbian Jewry.
Beginning in 1944, the town was part of the Autonomous Province of Vojvodina within the new Socialist Yugoslavia and, from 1945, within the Socialist Republic of Serbia. From 1992 to 2003 it was part of the Federal Republic of Yugoslavia, which was then transformed into the state union of Serbia and Montenegro. Since the 2006 independence of Montenegro, Sremska Mitrovica is part of an independent Serbia.