The group was first revealed at the end of September 2016. Set to be a 12-member group, one new member would be revealed per month until their debut in 2018, corresponding to their Korean name Idarui Sonyeo, or "Girl of the Month". Each new member revealed released a solo song and sometimes also a new song with members who have already been revealed. For their solo releases, the first five members traveled to locations such as Paris, Tokyo, Iceland, London, Taiwan, Hong Kong, New Zealand, and Prague for their promotions. Three sub-units were also revealed before their debut: LOOΠΔ 1/3, ODD EYE CIRCLE, and yyxy.
On August 7, 2018, LOONA released the digital single "favOriTe" as the lead-in single for their full debut. They held their debut concert "LOONAbirth" on August 19th and released their debut mini-album + + the following day. It debuted at #2 on South Korea's Gaon Album Chart, and was the second best-selling debut album by a girl group in 2018. [+ +] was repackaged as [× ×] on February 19, 2019, with six additional tracks, including a new single, "Butterfly". On August 17th, LOONA made their first appearance in the United States, performing at KCON 2019 in Los Angeles.
On February 5, 2020, LOONA released their second mini-album titled [#]. The mini-album also debuted at #2 on the Gaon Album Chart, and on March 12th, the group won their first music show trophy with lead track "So What" on Mnet's M!Countdown. On October 19th, LOONA released their third mini-album, [12:00] which became their best selling album to date. The mini-album debuted at #4 on the Gaon Album Chart, and is their first album to enter the Billboard 200 at #112. On November 18th, they released the music video for "Star", another song from their third mini-album. The song peaked at #40, and became their first entry on the Billboard Mainstream Top 40, making them the second K-pop girl group to enter the chart.
Their fourth mini-album, [&], is released this week on June 28th.