After completing his apprenticeship as a bookseller, Karl Günther, son of a Viennese printer, joined Alois Reichmann's company in 1922. Even in the 1920s he was a member of völkisch nationalist groups and of the Deutscher und Österreichischer Alpenverein (DÖAV), renamed Österreichischer Alpenklub in 1927, and the Sudetendeutscher Heimatbund. He was an active member of the illegal NSDAP from September 1934, keeping leaflets and arms at his home. In 1934 he also joined the SA. In August 1937 he became head of the culture department in Vienna's 4th district on behalf of the illegal NSDAP Gau cultural department. After the annexation, he was given the low NSDAP membership number 6241944. He was able to use the contacts formed while the Party was banned as a springboard for his career. He was an economic consultant in the Alte Wieden NSDAP section, from September 1938 an NSDAP culture consultant in District III and Truppführer (troop leader) of SA Standarte 10/4. On 15 April 1938 he became temporary administrator of Felix Reichmann's bookstore and in this function transferred books from there to NSDAP organizations. After the Aryanization application of the bookseller Johannes Katzler prevailed over Günther's own application and Katzler Aryanized the company, Günther resigned. He worked from summer 1938 as a consultant in the antiques group of the Reichsschrifttumskammer (Reich Literature Chamber) in Vienna and with a letter of recommendation from the Chamber and the officer in the SD and SS responsible, he was granted permission in December 1938 by the Vermögensverkehrsstelle (Property Transaction Office) to Aryanize Buchhandlung Zentralantiquariat Moritz Stern at Mariahilferstraße 1 in Vienna's 6th district. When he was conscripted in 1940, his wife continued to manage the company until April 1945. After his return from the war he moved back to Donnersbachwald in the district of Liezen, Styria.
The first impetus for criminal prosecution came in April 1946 from Heinrich Weisshappel of the Korporation der Wiener Buch-, Kunst- und Musikalienhändler (Corporation of Viennese Book Sellers and Art and Music Dealers), who wrote a report to the city council. Günther was arrested in Donnersbachwald in September 1946 and arraigned before the Vienna Landgericht (provincial court). In April 1947 he was tried before the Vienna Volksgericht for registry fraud (§ 8 of the Prohibition Act – VG), high treason §§ 10 and 11), illegal enrichment (§ 6 of the War Criminals Act – KVG) and denunciation (§ 7). The Volksgericht sentenced him in June 1947 to three and a half years' imprisonment for violations of § 11 (10) VG and und § 7 KVG and the forfeiture of his entire assets. By contrast, he was acquitted of illegal enrichment through the Aryanization of Antiquariat Moritz Stern. He obtained an early release from prison in January 1949. After his sentence was deemed to have been completed under the Amnesty Act of May 1957, he attempted unsuccessfully to have his forfeited assets returned to him.