Schonthal was born in Hamburg in 1924 to a Jewish family and began composing at age five, becoming the youngest student ever accepted at the Stern Conservatory in Berlin, where she studied piano and theory. In 1935, she was expelled from the country as anti-Semitism took hold; her family soon immigrated to Sweden and she entered the Royal Academy of Music in Stockholm.

Later, the family was again forced to relocate and moved to Mexico City in 1941, where Schonthal studied with Manuel Ponce. At age 19, she was the soloist in the world premiere of her Piano Concerto at the Palacio de Bellas Artes. In 1946 she met Paul Hindemith and moved to the U.S. to study with him at Yale as a scholarship student. She graduated with honors in 1948.

She wrote song cycle settings of poets including Yeats, Wordsworth, Dickinson and Garc‹a Lorca. Her chamber works included the 1994 piece The Wall Before and After, about the reunification of Berlin; the 1997 Bells of Sarajevo for clarinet and prepared piano; and the 1994 Fantasy-Variations on a Jewish Liturgical Theme for electric guitar. Orchestral works included the 1981 The Beautiful Days of Aranjuez for harp and strings. Her three operas were The Courtship of Camilla, Princess Maleen and Jocasta, written in 1979, 1989 and 1997, respectively.

Schonthal was better known in Europe than the U.S. In 1980 she visited and performed in Germany for the first time in more than forty years; she was awarded a medal by the city of Heidelberg in 1994. She was on the composition faculty at New York University and the Westchester Conservatory of Music.

According to the Times, the cause of death was a heart attack. She died at her home in Scarsdale, New York.