On December 8, 2016, she was revealed as the third member of LOONA. Her solo single album, titled "HaSeul", was released on December 15.
2017: Debut with LOONA 1/3, MIXNINE
She debuted as a member of the group's first sub-unit, LOONA 1/3, on March 7, 2017 with the mini album Love & Live.
In October, HaSeul, alongside fellow members HeeJin and HyunJin, auditioned for the survival show MIXNINE. However, HaSeul did not pass the audition.
2018: Debut with LOONA
On August 20, 2018, LOONA debuted as a full twelve-member group with their first mini album + +.
On January 8, 2020, BlockBerryCreative released a statement stating that HaSeul had been diagnosed with recurring anxiety. In order to recover, it has been decided that she will be taking a temporary hiatus to focus on her mental health. Due to this, she will not be promoting in the then upcoming album #.