The future jurist and art collector Walther Kastner was born in Gmunden in 1902 and grew up and attended school in Linz. After breaking off the study of art history, German and psychology at the University of Vienna, he joined the Bank für Oberösterreich und Salzburg but was dismissed in 1925 on account of the economic situation. He then studied law in Innsbruck and returned to Vienna to complete his doctorate. He did his court training at the local court in Schiffamtsgasse in the 2nd district before moving in 1930 to the Finanzprokuratur (Financial Procurator's Office). He started to build up a private art collection specializing in graphic art and acquiring works by Alfred Kubin, Lovis Corvinth, Max Liebermann and Max Beckmann. Shortly after the annexation of Austria in 1938, he was asked by Arthur Seyß-Inquart to head the legislation and legal alignment/economic law department in the Ministry of Internal and Cultural Affairs. He claims to have refused to join the NSDAP at that time and agreed to do so only in 1942, joining the Party the following year. According to archive material, however, Kastner's first application to join the NSDAP was made in 1940. The application included several positive assessments, but Kastner was not accepted immediately. It was not until he applied again in 1942 that he was accepted (membership number 9,021.901). After just a few months in the ministry, Kastner became Prokurist in October 1938 at the Österreichische Kontrollbank, an institution that played an important role during the Nazi era in the Aryanization of large Austrian companies. In his 1982 autobiography, Kastner wrote that the Kontrollbank handled 101 Aryanizations, a figure confirmed by the documents available. In 1942, Kastner left the Kontrollbank, which was liquidated after the Aryanizations had been completed. He then joined the board of Semperit AG, later becoming chairman and remaining in this function until the end of the war.
In May 1945, Kastner, as a former member of the NSDAP, was put in a penal work detail. He belonged to the art salvaging troop and from 1946 he was employed as a labourer in Stift Klosterneuburg. He appeared before the Volksgericht in Vienna on charges of enrichment (Section 6 of the War Criminals Act) in connection with two Aryanizations handled by the Kontrollbank. The proceedings in both cases were discontinued in 1946 and 1949, respectively. By this time Kastner had already managed to reintegrate into society. He worked from 1946 as a jurist for the "Krauland ministry", the Bundesministerium für Vermögenssicherung und Wirtschaftsplanung (Federal Ministry for Securing Property and Economic Planning) headed by Peter Krauland (ÖVP). Initially Kastner was involved with nationalization legislation, later in particular with restitution legislation. He drafted the Fifth Restitution Act (BGBl. 1949/164) on his own responsibility. Thus, from being responsible for Aryanization, he now handled restitutions, not only the judicial aspects but also in some cases their specific processing. He was responsible, for example, for the restitution of Bunzl-Biach AG, a company which he himself had Aryanized while working at the Kontrollbank. This strange constellation was no exception in post-war Austria. In 1947 Kastner opened a legal practice and also used his restitution expertise to represent Jewish clients dispossessed by the Nazis. Apart from his work as a consultant at the ministry, he also sat on numerous supervisory boards. In 1964 he was given the chair of the Department of Commercial and Exchange Law at the University of Vienna. As his career progressed in the 1950s, he also turned again to his art collection. According to Kastner, much of the original collection had been destroyed in an air raid in 1945. He specialized now in nineteenth-century Austrian art and acquired objects mostly from auctions at the Dorotheum in Vienna. In 1975 Walther Kastner gave most of his art collection to the Oberösterreichisches Landesmuseum (Upper Austria Provincial Museum). It comprised around 320 objects and was valued at 20 million schillings. The donation was added to subsequently by Kastner until his death in 1994 and then by his widow Franziska Kastner (1913–2008). In connection with an honorary doctorate awarded by the University of Vienna in 1992, questions were raised for the first time about Kastner's Nazi past, and some of the university senate, albeit a minority, voted against the award. During the discussion in the late 1990s of Nazi art looting and restitution policy, suspicion finally fell on Kastner's collection. The issue was examined in a research project by the Oberösterreichisches Landesmuseum. It concluded that the vast majority of Kastner's collection had been acquired after the war and that there was no direct link between his Aryanization activities at the Kontrollbank and his art collection. There were nevertheless several works purchased by Kastner from the Dorotheum, dealers and collectors in the 1950s and 1960s that had been expropriated or belonged to Nazi persecutees. To date, one painting has been restituted to the heir of Oskar Reichel. Research is being carried out on the restitution of other items. Details can be found on the website of the Oberösterreichisches Landesmuseum.