Cilmi was born 10 October 1991 in the Melbourne suburb of Dandenong. She lives with her dog Toni, brother Joseph and parents Paula and Joe. She attended Sacred Heart Girls' College, but now continues her education via correspondence. At a young age she acquired an interest in a wide range of musical styles ranging from Nina Simone to Led Zeppelin. Despite her natural aptitude and vocal talent, she was told that she lacked the discipline necessary to pursue a singing career. During this time she formed a garage rock band which performed cover songs of classic bands. She recorded original songs with writer/producers Barbara and Adrian Hannan of The SongStore.[2]

In 2004 she captured the attention of Michael Parisi, a record company executive from Warner Music, while giving an impromptu rendition of the Rolling Stones' song "Jumping Jack Flash" at a the Lygon Street Festa, a community festival in Melbourne. At the age of thirteen, Cilmi travelled to the U.S.A. and U.K. accompanied by Adrian Hannan and was offered four major U.S. deals. She eventually signed a contract with Island Records U.K.