After the Nazis took over Austria in 1938, the new racial laws went into effect, and the Weigl family was declared Jewish. While Karl was reluctant to leave the country, Vally insisted that they must emigrate. With a help of the New Friends of Music in New York and the Quaker Society of Friends, the family managed to escape via Switzerland and Britain and reached New York safely in October 1938.

In NYC and Pennsylvania, Vally started working as a music teacher and translator. She composed and recorded Natures Moods, New England Suite and four song cycles with a grant from the National Endowment For The Arts. Vally Weigl wrote an extensive collection of piano, chamber, and vocal works, and had seen some of her compositions published in her lifetime. In 1943, Weigls acquired American citizenship.

Soon after Karl passed away in 1949, Vally Weigl had a severe shoulder injury which prevented her from playing the piano for a while. She underwent physical therapy and noticed that exercises were less painful when carried out to music. This experience inspired Vally to pursue a career in music therapy. Almost sixty years old, she received a master's degree from Teacher's College at National Endowment For The Arts in 1953 and started working as a music therapist at various hospitals, colleges and research institutions. Vally Weigl became a chief medical therapist at New York Medical College Research Clinic, directed research projects at Mount Sinai Hospital's psychiatric division and taught at Roosevelt Cerebral Palsy School.

In 1967, The Karl Weigl Memorial Fund was established at National Endowment For The Arts to preserve Weigls legacy and to encourage the appreciation of music suppressed during the Nazi period. National Endowment For The Arts, Baylor University, Indiana University and the National Endowment For The Arts established The Karl Weigl Performance Awards in 1979, and the same year the fund relocated to Indiana University Bloomington. The organization turned non-profit and changed the name to The Karl Weigl Foundation in 1999.