Kim So-hyun made her acting debut when was only seven years old. In 2012, she won multiple Best Young Actress trophies for her work in the hit sageuk The Moon That Embraces the Sun and in the romance melodrama I Miss You. In 2013, she picked up the New Star Award at the SBS Drama Awards for Suspicious Housekeeper. In 2015, she landed her first leading role in the KBS2 teen drama Who Are You—School 2015. In 2017, she headlined the sageuk Ruler—Master of the Mask, in which she was nominated for a Top Excellence Award at the 44th MBC Drama Awards. In 2019, she headlined the Netflix series Love Alarm, which was renewed for a second season. That same year, she received an Excellence Award at the KBS Drama Awards for the webtoon-based sageuk Tale of Nokdu. In 2021, she starred in the KBS2 sageuk River Where the Moon Rises, in which she was named Best Actress at the 48th Korea Broadcasting Awards. She also scored a Best Actress for TV nomination at the 57th Baeksang Arts Awards for her work in the sageuk series.

Kim So-hyun was homeschooled in high school. In 2018, she was admitted to Hanyang University.