Hermann (von) Trenkwald was the son of Josef Matthias Trenkwald (1824–1897), a history painter and professor at the Akademie der bildenden Künste in Wien (Academy of Fine Arts in Vienna), who was ennobled in 1895, and his first wife Kate Noel (1838–1866), who was from Britain. After completing school at the Theresianum, he studied law in Jena and Vienna and after completion of the coursework (absolutorium) in 1891 he began to study art history, archaeology and history at the University of Vienna, where he obtained a doctorate in 1894 with a dissertation on the depiction of Christian virtues in fine arts. He worked thereafter first as a volunteer at the Königliche Museen in Berlin and then as a curator at the newly founded Kaiser Franz Josef-Museum für Kunst und Gewerbe in Troppau (Emperor Franz Josef Museum of Art and Industry in Opava), and finally as director from 1897 of the Kunstgewerbemuseum (Arts and Crafts Museum) in Frankfurt am Main. After serving in the First World War, he became first deputy director of the Österreichisches Museum für Kunst und Industrie (now the MAK) in Vienna. He became director at the end of 1925 before retiring prematurely, officially for health reasons but in fact on account of accusations of irregularities, in mid‑1927. Apart from his museum work, he also became artistic director of the Wiener Gobelinmanufaktur in 1921. Trenkwald wrote and edited specialist publications and exhibition and auction catalogues and gave talks. Between 1932 and 1935 he had his own art consultancy office in Palais Pallavicini on Josefsplatz in Vienna.
Trenkwald was admitted to the NSDAP on 1 May 1938 (membership number 6,150.369) but must effectively have been a member of the Party since 1931/32. From 1938 he played an active role in the Nazi Aryanization policy and was a fanatical antisemite. As a leading figure in the art commission within the Vermögensverkehrsstelle (Property Transaction Office) he was closely involved in the expropriation of collections and art and antiques dealerships owned by Jews. In 1939 he proposed the creation of a trusteeship for the sale of private "non-Aryan" art and suggested that important cultural objects should not be allowed to go abroad or even to the Old Reich. After Trenkwald's death in 1942, his widow Theresia Trenkwald (1893–1963) moved her antiques dealership from Fürstenstraße 1 in Maria Enzersdorf (then the 24th district of Vienna) to her residence at Stock-im-Eisen-Platz 3 in the 1st district. From 1950 until her death in 1963 she continued to deal in antiques, pictures and art objects at Getreidemarkt 2 in the 1st district. The present-day Wien Museum acquired five oil paintings from her, including a work by Johann Michael Neder, the oil painting Self-Portrait in Profile with Artist's Palette by Koloman Moser and the picture Melanie Schiele by Egon Schiele. The provenance has not yet been established.