Pre-debut: Formation through Idol School and "Glass Shoes"

In March 2017, it was announced that Mnet, same producer of shows including Sixteen and Produce 101, would be launching a new reality survival show titled Idol School to form a new girl group.[6] The show premiered on July 13 and ended on September 29, 2017 with Roh Ji-sun, Song Ha-young, Lee Sae-rom, Lee Chae-young, Lee Na-gyung, Park Ji-won, Lee Seo-yeon, Baek Ji-heon and Jang Gyu-ri as the nine members of Fromis. The final line-up was solely decided through live and online votes of viewers.Pledis Entertainment led by CEO Han Sung-soo managed the group's training and debut.

Fromis premiered its own reality program on October 19 titled Fromis's Room about continuing the path to debut. It followed the format of a simulcast, where it combined pre-recorded materials and replays of FaceBook Live broadcasts in their dorm.The group's name was later changed to Fromis 9 following the final episode of Fromis's Room on November 23, with the "9" meaning the nine students who graduated from Idol School.

On November 29, Fromis 9 performed their pre-debut single titled "Glass Shoes" at the 2017 Mnet Asian Music Awards in Japan. The song was released as a digital single the following day.They also performed the song on the December 15th episode of Music Bank, the group's first-ever appearance in a music program.

2018: Debut with To. Heart and To. Day

On January 8, 2018, it was announced that Fromis 9 would officially debut with their first extended play (EP) titled To. Heart.The EP, along with its lead single titled "To Heart", was released on January 24.The EP debuted at number 4 on the Gaon Album Chart issued on January 27, 2018.

On May 10, it was confirmed that Jang Gyu-ri has entered as a contestant in Produce 48 during M Countdown.Fromis 9 remained as an eight-member group and released their second EP, To. Day, on June 5, without Jang Gyu-ri due to her participation in the show.Fromis 9 went back to being a nine-member group after Jang Gyu-ri was eliminated in the 3rd elimination as she ranked at 25th place.

Starting from September 21, Fromis 9 would be managed under Off The Record Entertainment, a new agency exclusively established for Fromis 9 and South Korean-Japanese girl group Iz One.

The group has released a special single album titled From.9 on October 10 with the title song being LOVE BOMB, which reached 1 million views on YouTube on October 12. This was the first comeback with all the 9 members after the return of Jang Gyu-ri, who participated on Mnet’s Produce 48.

On October 15, 2018, Fromis_9 made their acting debut in a web series Welcome to Heal Inn on their official VLive page. However, it was filmed during Gyu-ri's absence.

2019: Fun Factory

On February 8, 2019, a seconds short season in announced for the Winter Season, this time with Jang Gyu-ri added as a new traveller. In May, Jang Gyu-ri made her solo acting debut in the web drama, Compulsory Dating Education.

On May 22, it was revealed that Fromis 9 will make their comeback on June 4.[28] After the announcement, Fromis 9 shared a prologue for their first single album, Fun Factory.