Born Margarete Feiglstock to a Jewish family in Vienna, Forst made her operatic debut in Cologne in 1900 in the title role of Lucia di Lammermoor Three years later, made her Vienna State Opera debut in the same role and was made a member of the company by Gustav Mahler. In 1908 she sang in the premiere of Karl Goldmark's Ein Wintermärchen with Leopold Demuth.
She remained in Vienna singing coloratura roles such as Olympia, Queen of the Night, Oscar, and Fiordiligi, as well as lyric soprano roles such as Cio-Cio-San. After she retired in 1911 upon her marriage to banker Johann Schuschny, she continued her career as a concert singer and teacher in Vienna for many years. She had one child, a son, Fritz Schuschny.
- Biography Archived 2010-12-19 at the Wayback Machine on operanederland.nl (in Dutch)
- Donald Mitchell Gustav Mahler: the Wunderhorn years : chronicles and commentaries 1980 p404 "These were the most noteworthy productions in the first year. Mahler brought a host of distinguished artists to the Opera: Mildenburg, Gutheil-Schoder, Kurz, Foerster-Lauterer, Weidt, Forst, Weidemann, Slezak, Mayr, to name only a few."
- Israelitische Kultusgemeinde Wien records
- Morgenstern, Hans (2009). "Forst, Grete", Jüdisches biographisches Lexikon, p. 238. LIT Verlag Münster. ISBN 3-7000-0703-5. This information also appears on a List of murdered Jews from Austria found in Namentliche Erfassung der oesterreichischen Holocaustopfer, Dokumentationsarchiv des oesterreichischen Widerstandes (Documentation Centre for Austrian Resistance), Wien. She is listed under her married name: Margarete Schuschny.
- His Master's Voice: the French catalogue Alan Kelly, Gramophone Company, EMI Music Archive 1990 p226 (Massé), pt I pt 2 LUCILLE MARCEL Obstination (de Fontenailles) GRETE FORST Nymphes et sylvains (Seulberg) Les Noces de Jeanette: Air du rossignol avec flûte
- Alan Blyth, Malcolm Walker - 1984 "Leo Slezak, with Grete Forst, is hypnotically beautiful, but are a dozen honeyed repetitions of 'Seligkeit', lovingly crooned, quite what this duet, in substance, is?"