The county covered 3,337 km2 (1,288 sq mi) and was located in the central part of Greater Romania, in the southeast of Transylvania.

The territory of Trei Scaune County was transferred to Romania from Hungary as successor state to Austria-Hungary in 1920 under the Treaty of Trianon.

In 1938, King Carol II promulgated a new Constitution, and subsequently he had the administrative division of the Romanian territory changed.

In September–October 1944, Romanian forces with Soviet assistance recaptured the ceded territory and reintegrated it into Romania, re-establishing the county.

According to the 1930 census, the county counted 136,122 inhabitants, of which 80.4% were Hungarians, 16.0% Romanians, 2.2% Romanies, 0.6% Germans, 0.5% Jews, and other smaller minorities.