He remained active in the underground Party as an ideological officer (1935-1937) and, from October 1937, leader (Führer) of the illegal SA-Brigade 5, "Mittel-Steiermark", based in Graz.

[2] Following Austria's Anschluss with Nazi Germany on 12 March 1938, Uiberreither was promoted to SA-Brigadeführer and was named Acting Police President for Graz.

Additionally, on 9 June he was named Landeshauptmann of Styria, thus uniting under his control the highest party and governmental offices in his jurisdiction.

The federal States were abolished and the country was divided into seven Reichsgaue, each headed by a Reichsstatthalter (Reich Governor) reporting directly to Hitler.

On 14 April 1941, after the conquest of Yugoslavia, he was named the Chief of Civil Administration in the occupied area of Lower Styria (now part of Slovenia).

He convened a meeting on 6 May 1941 at Maribor to plan measures that resulted in the expulsion to Serbia of tens of thousands of Slovenes in three waves beginning in July 1941.

[6] In addition, the Aktion T4 program of euthanasia involving mentally impaired individuals was operational within Styria and Lower Slovenia at this time.

Uiberreither was also charged with construction of defensive positions against the anticipated assault by the Red Army, which involved the procurement of forced civilian labor.

[10] In May 1945, after the Allies had overrun the Reich and Hitler was dead, Uiberreither was arrested in Murau by British authorities and later testified as a defense witness at the Nuremberg Trials.