Kim So-jin started her acting career on stage before appearing in television and films. Her early notable works as a movie star are the movies The Terror Live, No Tears for the Dead, and The Divine Move. In 2014, she played her first starring role in the drama film See, Beethoven. In 2016, she made her television debut in the MBC series Marriage Contract. Her breakthrough came in 2017 when she portrayed a tenacious prosecutor in the political crime drama film The King. Her performance in the movie won her numerous Best Supporting Actress citations, including one from the 53rd Baeksang Arts Awards. In 2019, she starred in the drama film Another Child, which earned her acting nominations at the 56th Grand Bell Awards and the 56th Baeksang Arts Awards. In 2020, she appeared as a lobbyist in the film The Man Standing Next, which scored her nominations for Best Supporting Actress at the 25th Chunsa Film Art Awards and the 29th Buil Film Awards.

Kim So-jin studied at Chung-Ang University and the Korea National University of Arts.