Pre-Debut

In February 2015, MBK Entertainment announced its plans to debut a new girl group. Originally, the company had decided to have possible candidates for their new group compete on a reality survival program, through which the public could become familiar with the group members.

MBK Entertainment revealed the first potential members to be Huihyeon, Eunjin and Kim Minhyun. Minhyun later left the project due to personal reasons, and Moon Seulgi and Chaeyeon were added to replace her. On February 10, Kim Dani was confirmed to be joining the project and announced the program's name "T-ara's Little Sister Girl Group". An additional member, Seunghee was also confirmed to have joined the lineup. Seunghee originally made her debut as member of F-ve Dolls in 2013, however she left two years later when the group disbanded.In June, MBK announced that they had cancelled their plans for a survival program, and decided to select the members internally. The final three members, Yebin, Eunice and Jenny were revealed and the group as set to debut in August under the name "DIA".MBK announced a line-up with six members: Eunice, Huihyeon (originally under the stage name Cathy), Jenny, Yebin, Eunjin and Chaeyeon.Seunghee was added to the line-up before debut.

Debut with "Do It Amazing"

On September 14, 2015, DIA released their self-titled debut album, Do It Amazing. On the same day, the music video for their lead single, "Somehow", was released. Their first official public appearance was also on September 14; the group performed a showcase at the Ilchi Art Hall in Seoul. They made their official debut stage on the music program M! Countdown on September 17, 2015.

On October 19, DIA released the music video for "My Friend's Boyfriend", another song from their debut album, and they continued their promotions for this new song on music stages.

On December 17, Cathy and Chaeyeon were announced to have temporarily withdrawn from DIA to join Mnet's Girl Group Project Produce 101. It was revealed that both Cathy and Chaeyeon auditioned and signed contracts with the show before DIA's official debut. DIA was said to continue to promote as a five-member group for the time being. After being eliminated in episode 11, Cathy resumed her activities with the group. Chaeyeon placed 7th in the final episode which granted her entrance to I.O.I, and she continued her hiatus for ten months to carry out her activities with I.O.I when the project group Debuted in May.

Line-up changes; Happy ending & Spell

On March 7, MBK Entertainment announced the addition of a new member, Eunchae (introduced with her real name Chaewon) and confirmed they were currently preparing for a comeback.

On April 13, MBK Entertainment announced that Seunghee's contract with the company would come to an end on April 30, and she decided not to renew. On April 19, I.O.I's agency stated each girl's contracts allowed for the possibility of activities under their respective companies while under the group, however indicating that "I.O.I will be promoting as a whole or in units for the upcoming 10 months". On May 11, MBK Entertainment later confirmed that Chaeyeon and Cathy will return to the group for upcoming comeback.It was confirmed that Cathy will be used her real name Ki Hui-hyeon and Chaewon will be used stage name Eunchae for future promotions. On May 28, DIA performed at Youth Dance Festival with Eunchae for first time.

DIA's first mini album Happy Ending was released on June 14. The group also released the music video for lead single "On The Road" on the same day, and had their comeback stage on M Countdown on June 16.

On August 11, MBK Entertainment released a teaser for their Harry Potter-themed comeback. The group held a showcase for their second mini album Spell at Blue Square Samsung Card Hall on September 12. The title track Mr. Potter and mini album were available for digital purchase on September 13, and physical purchase on September 21.