Rainer was a native of Sankt Veit an der Glan in Carinthia, the son of a German nationalist vocational teacher at a municipal Bürgerschule (secondary school).

He attended the Realgymnasium in Klagenfurt and, having obtained his Matura degree, studied law at the University of Graz while he earned his living by working in a local banking institution or in general labour.

As the Nazi Party had been banned by the Austrian government under Chancellor Engelbert Dollfuss in 1933, Rainer was in August 1935 sentenced to one year in police custody, presumably for high treason.

He was released early for good behaviour the following March, nevertheless like Klausner and his deputy Globocnik he had to step down from his administrative role in the party, transferring sole leadership to the rival Austrian Nazi leader Josef Leopold.

On 15 March 1940, he was additionally appointed as the Reichsstatthalter (Reich Governor) of Salzburg, thus uniting under his control the highest party and governmental offices in his jurisdiction.

[6] On 27 November 1941, Rainer was appointed as the Gauleiter and Reichsstatthalter of Reichsgau Carinthia, which also involved ruling over the adjacent occupied Yugoslavian territories in Upper Carniola.

On 7 May 1945, eight days after Hitler's suicide, Rainer transferred his official functions to an executive board and fled to the mountainous area around the Weißensee lake in Carinthia.

An entry in the diary of Boris Kraigher, former interior minister in Slovenia, indicates that Dr Rainer was executed with a number of other prisoners in late November 1950.