She studied singing with Lilli Lehmann and made her debut in 1917 at the Berlin State Opera (BSO) as Konstanze in Mozart's Die Entführung aus dem Serail.
She continued to excel in portrayals of heroines in Mozart operas at the BSO and at the Salzburg Festival; making her first appearances at the latter institution in 1922 as Susanna in The Marriage of Figaro and Zerlina in Don Giovanni.
She appeared in several United States premieres at the Met, including Lisette in Puccini's La rondine (1928), Aithra in Richard Strauss' Die ägyptische Helena (1928), Yvonne in Ernst Krenek's Jonny spielt auf (1929), Volkhova in Nikolai Rimsky-Korsakov's Sadko (1930), and Sofia in Rossini's Il signor Bruschino.
Other roles she sang for the company included Adina in L'elisir d'amore, Alice Ford in Falstaff, Ännchen in Der Freischütz, Berthe in Le prophète, Dorabella in Così fan tutte, Eva in Die Meistersinger von Nürnberg, Fiametta in Boccaccio, the Forest Bird in Siegfried, both Gutrune and the 3rd Norn in Götterdämmerung, Hänsel, Lauretta in Gianni Schicchi, Marguerite in Faust, Marzelline, Mathilde in William Tell, Micaela in Carmen, Musetta in La bohème, Nedda in Pagliacci, Ortlinde in Die Walküre, Pamina in The Magic Flute, Petrita in Suppé's Donna Juanita, Princess Eudoxie in La Juive, Serpina in La serva padrona, the Shepherd in Tannhäuser, Sophie in Der Rosenkavalier, Stéphano in Roméo et Juliette, Woglinde in Das Rheingold and Zerlina.
From 1934 until her retirement in 1949, Fleischer was a leading soprano at the Teatro Colón in Buenos Aires where her husband, conductor Erich Engel, was the director.