Heo Nam-joon made his acting debut in the 2019 film Like the First. The following year, he landed his first small-screen role in the OCN fantasy-mystery drama Missing: The Other Side. In 2023, he joined the second season of the Netflix apocalyptic horror drama Sweet Home. In 2024, he acted alongside veteran actors Sohn Hyun-joo and Kim Myung-min in the ENA crime-thriller Your Honor. Both his roles in Sweet Home and Your Honor earned him a Best New Actor nomination at the 2024 APAN Star Awards.


Also in 2024, Heo Nam-joon played a psychiatrist in the MBC series When the Phone Rings, which won him Best New Actor at the MBC Drama Awards. In 2025, he appeared in tvN’s When the Stars Gossip.


Heo Nam-joon studied acting at the Sungkyunkwan University.