After leaving school, Josef Fleischner, son of a Moravian glassware factory administrator, trained to become a stenographer in the Austrian parliament and was employed there as an assistant from 1880. He quickly rose to become a parliamentary stenographer and reporter. In 1899 he was made director of the Stenography Bureau and began to militate energetically to improve the status of stenographers and even before the end of the century fought for their permanent employment. As director he was also responsible for publishing the Reichsrat-Korrespondenz. In this function he developed the magazine's committee reporting and ensured the rapid translation into Hungarian of the reports about the "delegations", the joint meetings of the Austrian and Hungarian parliaments. He also worked as a stenographer and journalist in numerous committees in the provincial parliaments for Lower Austria, Carinthia, Moravia and Silesia. During his term of office from 1880 to 1923 he took part in 3,600 meetings of the chambers of the Reichsrat of the First Republic. He retired in 1923 and was made section head by Federal President Michael Hainisch.

After the annexation of Austria to the German Reich, Josef and his wife Olga Fleischner, née Brass, were persecuted by the Nazis as Jews although they had converted to Catholicism in 1894 (Josef) and 1914 (Olga). In view of his dire situation, in June 1940 Fleischner offered a collection of letters to the Wiener Stadtbibliothek (Vienna City Library), including letters from Pauline Metternich-Sándor, Johann Chlumecky, Ottokar Czernin, Berta Suttner, Hans Wilczek, and a number of photographs. In October of that year the library acquired this collection, keeping the letters and giving the photographs to the Städtische Sammlungen Wien (Historisches Museum). The Fleischners were ordered to move to a "Sammelwohnung" (collective apartment) at Novaragasse 41/7 and deported on 20 August 1942 to Theresienstadt ghetto, where Olga perished on 30 December 1942 and Josef on 8 January 1943.

In 2002, all of the acquisitions from Josef Fleischner were returned by the Vienna City Library and Städtische Sammlungen to the legal successors pursuant to a decision of the Vienna Restitution Commission and then repurchased for the collections of the present-day Wien Bibliothek (Vienna Library) and Wien Museum.