Zwilich did not begin her composing career writing in this vein. Her earliest works, such as the Symposium for Orchestra (which Pierre Boulez conducted in 1975) or the String Quartet 1974, featured the angular contours and difficult harmonic language that one might expect from a student of Elliott Carter and Roger Sessions (both professors at Juilliard). Her music took a decidedly more accessible and emotional turn as the composer approached maturity -- due, in part, to the deep introspection that followed the death in 1979 of her husband, violinist Joseph Zwilich. Her career quickly took off during the subsequent decade. After the premiere of her Symphony No. 1 by Gunther Schuller and the American Composers Orchestra in 1982, she received a nearly continuous string of symphonic commissions from various orchestras, including the New York Philharmonic, the Chicago Symphony, and the Boston Symphony. These commissions resulted in three subsequent symphonies, the orchestral work Symbolon (1988), and an impressive collection of over a dozen concertos during the 1980s and 1990s. Zwilich also composed a number of well-received chamber works during this period, including a Concerto for trumpet and small ensemble (1984), as well as the String Quartet No. 2 (1998), commissioned by the Emerson Quartet. In the 1990s, Zwilich also undertook significant audience outreach projects, such as the Making Music concert and lecture series, which she organized, and, for Carnegie Hall's family concert series, the charming Peanuts Gallery (1997), a composition for piano and orchestra inspired by the beloved Charles Schulz comics.
Although to describe Zwilich as an outstanding woman composer would unjustly confine her success, she does deserve credit for opening a number of doors for female composers. She was the first woman to receive a doctoral degree in composition from the Juilliard School, the first woman to receive the Pulitzer Prize in music (for her Symphony No. 1), and was named the inaugural holder of the Composer's Chair at Carnegie Hall.