Billie Eilish Pirate Baird O'Connell was born in Los Angeles, California, on December 18, 2001. The singer's first middle name, Eilish, was originally meant to be her first name, while Pirate (proposed by her then four-year-old brother Finneas) was to be her sole middle name, until her uncle objected because, you know, pirates are bad. Baird is her mother's surname.

She first gained attention in 2015 when she uploaded the song "Ocean Eyes" to SoundCloud, which was subsequently released by the Interscope Records subsidiary Darkroom. The song was written and produced by her brother Finneas, with whom she collaborates on music and live shows.

"My first real song was about the zombie apocalypse actually," she told Harper's Bazaar, "which is weird because it's out and nobody knows that that's what it's about. It's called 'Fingers Crossed'. I literally just watched The Walking Dead and I took little lines from it. Just watch all of The Walking Dead and you'll find some things that are in my song and some episode titles that are in my song."

I like when you get mad
I guess I'm pretty glad that you're alone
You said she's scared of me?
I mean, I don't see what she sees
But maybe it's 'cause I'm wearing your cologne
I'm a bad guy

After the release of their album High as Hope, Florence + the Machine announced the album's then-upcoming supporting tour in late May of 2018, including opening acts such as Billie Eilish, Lizzo, and St. Vincent. In December of 2018, Billie cancelled several dates on the tour due to "international scheduling conflicts", likely due to her having her own tour going on at the same time as well as the recording of her debut album.

After her rise to stardom, Eilish found that going out in public wasn't like it used to be. As a result, one of her favorite colors to wear when she wants to be incognito is black. "Black is the only time I can go through the airport and be fine," Eilish said during an interview with Vanity Fair. According to the singer, being in the airport is normally like a meet and greet, "but if I'm wearing a black beanie, and a black shirt, and some black pants, and some black shoes, I'm pretty good."

Her debut album When We Fall Asleep, Where Do We Go? made her the first artist born in the 2000s to have a number one album in the United States.

All the good girls go to hell
'Cause even God herself has enemies
And once the water starts to rise
And heaven's out of sight
She'll want the devil on her team

The album was recorded in her brother Finneas' small bedroom studio in Highland Park, Los Angeles. The pair explained that they chose this recording location rather than a professional studio due to the bedroom's intimate and homely nature as well as the manner in which the bedroom affects vocals.

Eilish made the switch to a vegan diet in 2014 and said that the main reason she went vegan is that she wanted to reduce her impact on the environment and be kinder to animals. Eilish's all-time favorite vegan food is burritos, and she has admitted that she eats a lot of them.