Kim Jung-hyun made his acting debut with a leading role in the 2015 movie Overman, opposite Chae Seo-jin. His performance in the film earned him Best New Actor nominations at the 25th Buil Film Awards and the 22nd Chunsa Film Art Awards. In 2016, he landed his first small screen acting role in the SBS series Jealousy Incarnate. In 2017, he won Best New Actor at the MBC Drama Awards for his work in the sageuk Rebel: Thief Who Stole the People. That same year, he led the KBS2 teen series School 2017 along with Gugudan’s Kim Se-jung. The drama scored him numerous acting nominations, including Best New Actor at the 54th Baeksang Arts Awards.  In 2018, he starred in the MBC series Time but had to withdraw halfway through its run due to health reasons. In 2019, he made a comeback in the top-rating tvN series Crash Landing on You, in which he was nominated for Best Supporting Actor at the 7th APAN Star Awards. In 2020, he was cast as King Cheoljong in the hit tvN fusion sageuk Mr. Queen.

Kim Jung-hyun fulfilled his mandatory military service when he was 21 years old. In 2018, he received criticisms for his supposed rude behavior toward his leading lady Seohyun at the press conference for the drama Time. In 2021, it was reported that actress Seo Ye-ji, who was Kim Jung-hyun’s then girlfriend, allegedly manipulated the actor’s behavior when he worked on the said 2018 series. Kim Jung-hyun apologized for his behavior while Seo Ye-ji’s agency denied that she controlled the actor.