Kim Nam-gil made his acting debut in the 1999 KBS youth drama School 1. In 2008, he appeared in the movie Public Enemy Returns, which scored him a Best New Actor nomination at the 29th Blue Dragon Film Awards. That same year, he became part of the film Modern Boy, in which he was named Best New Actor at the 17th Korea Culture and Entertainment Awards. In 2009, he became a household name for his portrayal of the character of Bidam in the top-rating MBC sageuk Queen Seon-deok. His performance in the series won him an Excellence Award at the MBC Drama Awards as well as the Best New Actor trophy at the 46th Baeksang Arts Awards. In 2015, he was nominated for Best Actor at the 24th Buil Film Awards for The Shameless, which he starred in together with award-winning actress Jeon Do-yeon. In 2019, he led the hit SBS series The Fiery Priest, which gave him numerous acting trophies, including the Daesang at the SBS Drama Awards.

Kim Nam-gil used the stage name Lee Han in the early years of his career. He enlisted for his mandatory military service in July 2010 and was discharged in July 2012. In 2015, he founded the non-profit culture and arts organization Gilstory. In 2021, he launched the comprehensive entertainment company Gilstory ENT in partnership with the film production outfit Sanai Pictures.