When We All Fall Asleep, Where Do We Go? was released on March 29, 2019. Spotify launched a "multi-level campaign behind the album", creating a multi-media playlist and "new product features" that Spotify stated "allow for vertical video content, custom assets, and editorial storylines all with the goal of creating more meaningful and engaging context for [Eilish's] fans." In Los Angeles, Spotify set up a "pop-up enhanced album experience", which included different artwork and a "multi-sensory" experience of each track for fans. The album debuted at No. 1 on the Billboard 200 as well as on the UK Albums Chart, making Eilish the first artist born in the 2000s to have a No. 1 album in the United States, and the youngest female ever to have a No. 1 album in the United Kingdom. Upon the album's debut, Eilish broke the record for most simultaneously charting Hot 100 songs by a female artist, with 14, after every song from the album, excluding "Goodbye", charted on the Hot 100. The fifth single from the album, "Bad Guy", was released in conjunction with the album. A remix of the song featuring Justin Bieber was released in July 2019. In August, Bad Guy peaked at No. 1 in the US, ending Lil Nas X's record-breaking 19 weeks at No. 1 with "Old Town Road". She is the first artist born in the 2000s and the youngest artist since Lorde (with "Royals") to have a number-one single.
Eilish began her When We All Fall Asleep Tour at Coachella Festival in April 2019, with the tour concluding on November 17, 2019 in Mexico City. In August 2019, Eilish partnered with Apple Music for Music Lab: Remix Billie Eilish, part of Apple Stores' Music Lab sessions during which fans deconstruct her song "You Should See Me In A Crown" and learn how to create their own remix on Apple devices and GarageBand. On September 27, 2019, Eilish announced her Where Do We Go? World Tour via her Instagram page. The tour was set to begin in Miami on March 9, 2020 and conclude in London on July 27, 2020, but had a number of dates postponed due to the COVID-19 pandemic.
On November 7, 2019, Jack White's Third Man Records announced the label would be releasing an acoustic live album of Eilish's performance from the record label's Blue Room, exclusively sold on vinyl at Third Man retail locations in Nashville, Tennessee, and Detroit, Michigan. On November 13, 2019, she released her next single, "Everything I Wanted". On November 20, 2019, Eilish was nominated for six Grammy Awards including Record of the Year and Song of the Year for "Bad Guy" as well as Album of the Year and Best New Artist. At age 17, she became the youngest artist to be nominated in all four General Field categories. In the same month, Eilish was crowned 2019's Billboard Woman of the Year.
Eilish has wanted to direct her own music videos since she was 14 but was initially not given the opportunity due to lack of experience. In 2019, she made her directorial debut with the video for her song "Xanny".
She is the youngest person, second person ever, and first female to win the four main Grammy categories – Best New Artist, Record of the Year, Song of the Year and Album of the Year – in the same year. In 2019, Time placed her on their inaugural "Time 100 Next" list.
According to Kenneth Womack in 2019, Eilish had "staked her claim as the reigning queen of electropop" with her debut album and "been variously called out for being precocious and the poster child for late-adolescent angst. But in truth, she’s nothing of the sort."
In May 2019, Eilish appeared in a Calvin Klein advertisement, wherein she mentioned that she dresses in baggy clothes to prevent people from body-shaming her. In a March 2020 live show in Miami, as part of the Where Do We Go? Tour, Eilish premiered a short film entitled Not My Responsibility which addresses her experiences of body-shaming. Not My Responsibility was later uploaded to Eilish's YouTube channel in May 2020.
In April 2019, Eilish released clothing in collaboration with Takashi Murakami, inspired by her music video for "You Should See Me in a Crown", also directed and animated by Murakami, as well as a limited edition vinyl figure of herself from the video. Eilish also collaborated with Adobe Creative Cloud the same month for a series of advertisements as well as a social media art contest, where users would submit artwork with the hashtag "#BILLIExADOBE".
Eilish appeared in the debut of Calvin Klein's "I Speak My Truth In #MyCalvins" ad campaign in May 2019, as well as the Ad Council's "Seize the Awkward" campaign, a series of PSAs targeting mental health awareness. She fronted MCM Worldwide's fall 2019 advertising campaign in July 2019, and later that month, collaborated with Los Angeles-based clothing brand Freak City for a clothing line. Also in July 2019, she performed at a dinner hosted by Chanel on Shelter Island to celebrate the brand's pop-up yacht club.
In August 2019, Eilish partnered with Apple to allow Apple Store customers to experiment with her song "You Should See Me in a Crown" in Music Lab sessions in its stores. Eilish's collaboration with the clothing company Siberia Hills was met with controversy after it was revealed that the company had used plagiarized designs of fan art of the character Nozomi Tojo from Love Live!, drawn by artist Makoto Kurokawa, for Eilish's clothing line. The brand later clarified that Eilish herself had no knowledge of the plagiarism.
Eilish is a regular advocate on social media for animal rights and veganism, and among others has criticised the dairy, wool, and mink fur industries. In 2019 she won a PETA "Best Voice for Animals" award for her activism.