Davies was born on May 29, 1973, in Bristol, England. As a teen, she played guitar and sang in a rock band, also developing an interest in composition at the time. Rock and funk have continued to play a role in her music. Davies studied composition and French horn at the Colchester Institute of the University of Essex and then went on to the Guildhall School of Music and Drama in London, studying with Simon Bainbridge. She took further composition lessons with Simon Holt. A major breakthrough in Davies' early career was a win at the BBC Young Composers Competition in 1996. She earned a PhD at Royal Holloway, University of London, where she served for a time as the composer-in-residence. In the early 2000s, Davies began to receive commissions from major British orchestras, including the London Symphony, the Orchestra of the Swan, and several BBC orchestras. The collage-like, funky neon (2004) has been one of her most successful works. Her Christmas hath a darkness was commissioned by and performed at the Festival of Nine Lessons and Carols at King's College, Cambridge, in 2011. Another important commission came from the English National Opera in 2015; Davies responded with Between Worlds, a setting of a libretto by Nick Drake (not the acoustic rock songwriter).
More than 20 of Davies' works have been recorded, seven of them on the 2021 NMC release Tansy Davies: Nature. Davies has taught at the Royal Academy of Music in London and, as of the early 2020s, she was on the faculty at the University of Indiana Jacobs School of Music in the U.S. ~ James Manheim, Rovi