Prior to her debut, Kim Se-jung joined the SBS talent competition show K-pop Star 2 in 2012, but was eliminated. In January 2016, she participated in Mnet’s reality show Produce 101, where she finished in second place. She then became part of the show’s project girl group I.O.I. In June 2016, she debuted as a member of the girl group Gugudan with the extended play Act. 1 The Little Mermaid. In November 2016, she released her solo debut song Flower Way. In 2020, she dropped her first solo extended play titled Plant, which she wrote and composed herself.

Kim Se-jung made her acting debut with a leading role in the 2017 KBS2 series School 2017, which won her Best New Actress at the KBS Drama Awards. In 2019, she starred alongside Yeon Woo-jin in the KBS2 mystery rom-com Let Me Hear Your Song. In 2020, she joined the main cast of the hit OCN supernatural-action drama The Uncanny Encounter. In 2022, she was paired with Ahn Hyo-seop in the webtoon-based SBS series Business Proposal.

Kim Se-jung joined track and field competitions during her elementary and middle school years. She attended Indeogwon High School and was a member of its theater club. She continued her studies at Hanyang Women’s University.