Renner was born the 18th child of an ethnic German family of poor wine-growers in Unter-Tannowitz (present-day Dolní Dunajovice in the Czech Republic), then part of the Margraviate of Moravia, a crown land of the Austro-Hungarian Empire.

During these early years, he developed new perspectives on law — all the while cloaking his innovative ideas under a variety of pseudonyms (for example, Synopticus and Rudolf Springer) lest he lose his coveted post as parliamentary librarian.

On the nationality question, he upheld the so-called "personal autonomy" on the basis of which the super-national state should develop, and thereby influenced the agenda and tactics of the Social Democratic Party in dealing with it.

Even before the collapse of the Austro-Hungarian Monarchy, Renner had proposed a future union of the German parts of Austria with Germany, even using the word "Anschluss".

There Renner had to accept that this new Austria was prohibited any political association with Germany and he had to accept the loss of German-speaking South Tyrol and the German-speaking parts of Bohemia and Moravia where he himself was born; this forced him to give up his share in the parental farm if he, "the peasant proprietor who turned Marxist",[5] wanted to remain an Austrian government officer.

A wide range of social reforms were introduced by Renner's government, including unemployment insurance, paid holidays, the eight-hour workday, and regulations on the working conditions of miners, bakers, women, and children.

Having originally been a proponent of new German-Austria becoming a part of the democratic German Republic, he expected Nazism to be but a passing phenomenon not worse than the dictatorship of Dollfuss's and Schuschnigg's authoritarian one-party system.

[10] On 3 April, at the beginning of the Vienna offensive, Renner, then living in southern Lower Austria, established contact with the Soviets.

Seven days later Renner's cabinet took office, declared the independence of Austria from Nazi Germany and called for the creation of a democratic state along the lines of the First Austrian Republic.

[12] The British were particularly hostile,[12] and even American President Harry S. Truman, who believed that Renner was a trustworthy politician rather than a token front for the Kremlin,[15] denied him recognition.

"[18][19][20] For most of his life, Renner alternated between the political commitment of a social democrat and the analytical distance of an academic scholar.

[21] His and Otto Bauer's ideas about the legal protection of cultural minorities were taken up by the Jewish Bund, but fiercely denounced by Vladimir Lenin.