After suspending her studies for two years during World War II to become a civilian flight instructor in Muncie, Indiana,[3] Hillis received a bachelor of music degree in composition from Indiana University School of Music in 1947 and later studied conducting privately with Julius Herford and with Robert Shaw at the Juilliard School.

In 1992, Hillis conducted the Interlochen Arts Academy Orchestra and Choir as part of the Mozart Bicentennial Celebration in Alice Tully Hall in Lincoln Center, New York.

Prior to her death, Hillis bequeathed her personal collection of scores, books, and other memorabilia to the Chicago Symphony Orchestra's Rosenthal Archives.

Auditions began two weeks later, and on March 13 and 14, 1958, the Chicago Symphony Chorus made its subscription concert debut performing Mozart's Requiem with Bruno Walter conducting.

Under Hillis's leadership, the Chicago Symphony Chorus performed and recorded many of the major works in the choral symphonic repertoire, gave important world premieres, [which?]

Starting in the late 1970s, she worked actively as a guest conductor, leading performances of the National, San Francisco, Indianapolis, Milwaukee, Saint Paul, Baltimore, Minnesota, Columbus, Peoria, San Antonio, Spokane, and Oregon symphony orchestras; the New York Choral Society; the Los Angeles Master Chorale; the Gloria Dei Cantores; and the Santa Fe Opera.