Its territory is now mostly in western Ukraine, a smaller part in eastern Slovakia, and a very small area in Hungary.

In the aftermath of World War I, most of Ung county became part of newly formed Czechoslovakia, as recognized by the concerned states in the 1920 Treaty of Trianon.

The town of Záhony and the village of Győröcske remained in Hungary, which was merged into Szabolcs-Ung County in 1923.

In 1939, following the annexation of the remainder of Carpathian Ruthenia after Czechoslovakia became abolished, the rest of the territories became part of Hungary again, however those were assigned to the administrative branch offices of Ung.

After World War II, as the 1920 borders were restored, the westernmost part was returned to Czechoslovakia.