She worked writing ad copy, released one poorly received album of her own ("Childstar"), and collaborated with Bonnie Raitt, but was, by her own admission, at the end of her rope when a collaboration with Maurice White of Earth, Wind & Fire led to the smash hit "September" (1978). The song sold 10 million copies, firmly establishing her as a songwriter. Her other big hit with Earth, Wind & Fire was "Boogie Wonderland" (1979).
Some of her other successes included "Neutron Dance" (1983) by the Pointer Sisters and "What Have I Done to Deserve This?" (1987) by the Pet Shop Boys with Dusty Springfield. She won a Grammy for writing a track on the "Beverly Hills Cop" soundtrack album (1986), but her most impactful work was surely "I'll Be There for You," co-written by Willis in 1994 and recorded by the Rembrandts. As the "Friends" theme song (she called it "the whitest song" she'd ever written), it earned her an Emmy nomination.
She also co-wrote the 2005 Broadway musical adaptation of "The Color Purple," the music from which won her a second Grammy. A film of the musical is currently in the works.
In 2018, she was inducted into the Songwriters Hall of Fame.
Willis was known for her collection of pop culture memorabilia, which she housed in a pink home in L.A., and which she showed off online via her site Museum of Kitsch.
Willis is survived by her partner of 27 years, Prudence Fenton, her brother, and her sister.