On December 4, 2015, Starship Entertainment and Yuehua Entertainment launched a 12-member girl group called WJSN, where the Korean and Chinese members were divided into different subunits.
On December 10, the first unit, Wonder Unit, was announced (Cheng Xiao, Bona and Dayoung). On December 17, the second three-member unit (Xuan Yi, Eunseo and Yeoreum) was introduced: Joy Unit. On December 21, WJSN's Wonder and Joy Units released a Christmas cover of "All I Want For Christmas Is You" by Mariah Carey. On December 24, the third unit, Sweet Unit, was announced (Seola, Exy and Soobin). On December 31, the final unit was announced: Natural Unit (Mei Qi, Luda and Dawon).
Cosmic Girls photographed during the closing ceremony of Yonsei University on May 26, 2016.
WJSN debuted on February 25, 2016 with the release of their debut mini album Would You Like?, including the title tracks "Mo Mo Mo" and "Catch Me." The group held their debut stage on the February 25th episode of M Countdown with "Catch Me" and "Mo Mo Mo."
On July 11, Starship Entertainment confirmed that I.O.I's Yoo Yeon-jung would be joining the group. In August 2016, Seola, Exy, Soobin, Eunseo, Cheng Xiao, Yeoreum, and Dayoung teamed up with label mate Monsta X to form the unit "Y-Teen." Y-teen was a project unit group that promoted as CF models for KT’s phone fare service and would release EPs, music videos, and various entertainment content.
On August 17, WJSN released their second mini album The Secret with all 13 members.
2017: From. WJSN, first concert and Happy Moment
On January 4, WJSN released their third mini album From. WJSN. The EP contains six tracks including the title track "I Wish."
WJSN held their first solo concert, "Would You Like - Happy Moment," from May 19–20 at the Blue Square Samsung Card Hall. A total of 2,000 fans gathered in two days.
Cosmic Girls in 2017
WJSN's first full-length album, Happy Moment, was released on June 7, with a total of ten tracks including the lead single "Happy." The album ranked number 1 on Hanteo's daily and real-time charts after release.
On February 27, WJSN released their fourth mini album Dream Your Dream, which consists of six tracks including the lead single "Dreams Come True." Their comeback showcase was held at the Yes24 Live Hall on the same day as the album's release.
On June 1, WJSN and Weki Meki formed a project group called WJMK and released a digital single "Strong."
Two Chinese members (Mei Qi and Xuan Yi) participated in the Chinese version of survival show Produce 101 during the first half of 2018. The survival show concluded with Mei Qi ranking 1st and Xuan Yi ranking 2nd, making them both members of the temporary girl group Rocket Girls 101.
On September 19, WJSN returned with their fifth mini album WJ Please?. The Chinese members, Cheng Xiao, Mei Qi and Xuan Yi, did not participate in the comeback and the group continued as ten members. Mei Qi and Xuan Yi were preparing to debut with Rocket Girls 101, while Cheng Xiao was filming Legend of Awakening, a Chinese historical fantasy drama as the second lead actress, which would be her acting debut.
2019: WJ Stay?, second concert, For the Summer, and As You Wish
On January 8, WJSN released their sixth mini album WJ Stay?, which consists of seven tracks including the lead single "La La Love." The group continued to promote as ten members, with the Chinese members, Cheng Xiao, Mei Qi and Xuan Yi, unable to participate in the comeback due to prior commitments.
WJSN held their second solo concert, "Would You Stay - Secret Box," from March 2–3 at the Blue Square Imarket Hall.
WJSN released their special album For the Summer on June 4, which consists of five tracks including the lead single "Boogie Up."
In August 2019, WJSN held their first concert tour titled WJSN 1st Mini Live 2019 "Would You Like?" Zepp Tour in Japan, starting on August 17 in Zepp DiverCity in Tokyo.
WJSN released their seventh mini album As You Wish on November 19, which consists of seven tracks including the lead single "As You Wish."
2020: Third concert, Neverland, and first sub-unit
In 2020, WJSN had planned to hold their third concert titled WJSN Concert "Obliviate," starting on February 22, 2020 at the Olympic Hall in Seoul, South Korea and finishing on March 22, 2020 at the Toyosu PIT in Tokyo, Japan. However, due to the COVID-19 pandemic, the concert was postponed.
WJSN released their eighth mini album Neverland, including the title track "Butterfly," on June 9.
On September 23, WJSN announced the formation of the new sub-unit WJSN Chocome, featuring members Soobin, Luda, Yeoreum, and Dayoung. They released their debut single album Hmph!, and its title track with the same name on October 7.
WJSN released their ninth mini album Unnatural, with the title track of the same name on March 31.
On April 26, WJSN announced the formation of their second sub-unit WJSN The Black, featuring members Seola, Exy, Bona, and Eunseo, along with announcing their first single album My Attitude and its title track "Easy", which was released on May 12.