Margarethe Hassel worked from 1909 to 1927 as a bank teller at the Creditanstalt and then for other unidentified employers in Vienna. In August 1938 she applied to the Vermögensverkehrsstelle (Property Transaction Office) for authorization to acquire and Aryanize the stocking and knitwear business belonging to Eugen Kempfner, who was persecuted as a Jew. She claimed to have worked "for around two years in an auction house" after leaving the bank, although it is not clear to what she was referring exactly. After the NSDAP Wirtschaftsamt in the Vienna Gau stated that Hassel, whom Kempfner himself had named as applicant and whom he had possibly known when she worked at the Creditanstalt, claimed that she had supported the Dollfuß-Schuschnigg regime, the business was awarded to the confirmed Nazi Aloisia Fischer, whereas Grete Hassel lived "very withdrawn, so that it is not possible to find out further details about her." She lived with her sister Theodora at Dornbacherstraße 104 in the 17th district and worked from July 1942 to early 1943 for Frommherz Müller & Co in the 1st district, then until the end of 1944 for A. Rudolph, Band- u. Flechtwaren GesmbH in the 6th district, remaining in the successor company Vereinigte Seidenwarenfabriken Gebrüder Schiel AG until 1950.
In connection with provenance research, Hassel is mentioned as having provided the farewell letter from Elsa Klauber who lived with Fritz Grünbaum's wife Lilly Grünbaum after his arrest. In the accompanying letter to Klauber's daughter Annemarie, Hassel stated that the two women had met in 1940 through a mutual acquaintance, the former bank manager Felix Fleischmann (1874–1962). Fleischmann fled in March 1940 with his wife Elvira (née Almoslino, divorced Bárány, 1873–1945) to the USA but returned after the war to Vienna and married Grete Hassel in 1950, the year she retired. It is unknown what she acquired from Elsa Klauber before her deportation and passed on to her daughter Annemarie or whether she held items for Lilly Grünbaum in safe keeping.