Casals Istomin was born Marta Montañez Martínez in Humacao, Puerto Rico, to a family of amateur musicians.

[1] After graduating high school, Montañez was awarded a scholarship to attend the Mannes College of Music in New York City to study cello.

She graduated summa cum laude and joined Casals in France where she became an active participant in the Prades Festival.

[3] From 1990-1997, Casals Istomin was the general director of "Recontres Musicales d'Evian Festival International" in France.

[2] Casals Istomin, who is fluent in English, Spanish, French and Catalan, belongs to or has acted as spokeswoman for the following organizations:[2] On July 1, 1992, Casals Istomin was named President of the Manhattan School of Music, a position which she held until her retirement in October 2005.

She has also been decorated with the highest cultural honors by the governments of Puerto Rico, Spain, France and Germany.