Cornish was born March 9, 1975, in London and grew up nearby in Kent. From an early age, she took violin, piano, and composition lessons, and by 12, she had written a symphony. At about that time, she was a finalist on violin in the BBC Young Musician of the Year competition. Cornish attended Kent's Folkestone School for Girls. She went on to the Royal Northern College of Music, studying composition with Anthony Gilbert, and earned a master's degree at the Royal College of Music, winning major prizes at the two schools. For the first part of her career, Cornish wrote film and television scores as well as works in major classical genres. Her BAFTA Award came in 2005 for her work on the television program Five Children and It. In 2008, Cornish's tone poem Symphony was given a Special Distinction nod by ASCAP's Rudolf Nissim Prize jury.
In the 2000s decade, Cornish did orchestration work on various film scores in both Britain and the U.S., including that for Kung Fu Panda (2008). She composed the score for Fireflies in the Garden in 2011. In 2012, Cornish served as arranger and session leader on pop singer Joshua Radin's album Underwater. In 2014, she began issuing albums under her own name on the Innova label; the first of these was Duende. It was followed by Continuum (2015), Into Silence (2017), and Constellations (2018). These releases contained various Cornish chamber music compositions. Cornish became a member of the Academy of Motion Picture Arts and Sciences in 2019. In 2022, she released the album Sierra, which she also produced. ~ James Manheim, Rovi