Weigl was born in Vienna, the son of a bank official who was also a keen amateur musician. Alexander Zemlinsky took him as a private pupil in 1896. Weigl went to school at the Franz-Joseph-Gymnasium and graduated from there in 1899. After that, he continued his studies at the Vienna Music Academy, where he became a composition pupil of Robert Fuchs, and also enrolled at the University of Vienna, studying musicology under Guido Adler, with Anton Webern as his classmate. His only opera, Der Rattenfänger von Hameln, premiered in Vienna in 1932.
When the Nazis occupied Austria, in 1938, Weigl emigrated to the United States of America, together with his second wife, musician and composer Vally Weigl (née Pick), and his son. There, he obtained a number of increasingly important teaching posts: at the Hartt School of Music, at Brooklyn College, at the Boston Conservatory and, from 1948 on, at the Philadelphia Academy of Music. He died in New York after a prolonged battle with bone marrow cancer.
Weigl wrote many compositions including symphonies, chamber music pieces including string quartets, and songs for solo piano.
- Symphony No. 1 in E major, Op. 5 (1908)
- Symphony No. 2 in D minor, Op. 19 (1922)
- Symphony No. 3 (1931)
- Symphony No. 4 in F minor (1936)
- Symphony No. 5, "Apocalyptic Symphony" (1945)
- Symphony No. 6 in A minor (1947)
- Der 71. Psalm for Female choir and Orchestra (1901)
- Symphonisches Vorspiel zu einer Tragödie (1933)
- Music for the Young(Boy Scouts Overture) für kleines Orchester (1939)
- Drei Gesänge für hohe Frauenstimme und Orchester (1916)
- Phantastisches Intermezzo, 1922 (4th movement from the 2nd Symphony, to be performed as a separate work)
- Piano Concerto No. 1 for the Left Hand in E flat, 1924
- Violin Concerto in D major (1928)
- Piano Concerto No. 2 in F minor, Op. 21 (1931)
- Rhapsody for Piano and Orchestra (1940)
- Der Rattenfänger von Hameln Op. 24, Märchenspiel in vier Bildern (1932)
- Drei Gedichte von Lenau für achstimmigen gemischten Chor a cappella op. 6, 1909
- String Quartet No. 1 in C minor, Op. 20 (1905/1906)
- String Quartet No. 2 in E major, with Viola d'amore (1906)
- String Quartet No. 3 in A major, Op. 4 (1909)
- String Quartet No. 4 in d minor (1924) Op. Post.
- String Quartet No. 5 in G major, Op. 31 (1933)
- Fünf Lieder für eine hohe Singstimme und Klavier, Op. 23 (1911)
- Violin Sonata No. 1, Op. 16 (1923)
- String Quartet No. 6 in C (1939)
- String Quartet No. 7 in F minor (1941-2)
- String Quartet No. 8 in D major (1949)
- Two Pieces for cello and piano, Op. 33
- Minuet for cello and piano
- Piano Trio (1939)
- Works by or about Karl Weigl at Internet Archive
- Torres, Claude. "Karl and Vally Weigl Works and Discography". Archived from the original on 2007-12-26.
- String Quartet No.3, Op.4 Sound-bites and discussion
- Free scores by Karl Weigl at the International Music Score Library Project (IMSLP)
- The Karl Weigl Papers at Irving S. Gilmore Music Library, Yale University
- Article on Weigl's Fifth Symphony (Accessed 17 May 2011)