Billie Eilish Pirate Baird O'Connell was born in Los Angeles, California, to actress and teacher Maggie Baird and actor Patrick O'Connell. Her middle name, Eilish, was originally meant to be her first name, while Pirate was to have been her middle name. However, after her grandfather William Baird died while her mother was pregnant, her parents decided to name her Billie, after her grandfather.

Eilish started playing the ukulele at age six and joined the Los Angeles Children's Chorus at age eight. She also began taking dance classes and performed at talent shows. Her parents, both musicians, encouraged her and her older brother Finneas to explore various artistic interests, including music, dance, and acting.