Kang Nam-gil began acting as a child in 1966. He widened his popularity in 1986 when he was cast in the MBC series Under the Same Roof, where played the character of Yoon Bong-soo. He continued to become a household name with his portrayal of Kang Dal-soo in a series of MBC Best Theater episodes beginning in 1995. One of the said episodes titled The Trial of Dal-soo recorded a high rating of 25.7 percent. He then earned back-to-back Best Supporting Actor nominations at the 2006 and 2007 KBS Drama Awards for Pure in Heart and Capital Scandal. In 2009, he appeared in the MBC romantic comedy Creating Destiny, which won him a Golden Acting Award at the MBC Drama Awards. He also received a Best Supporting Actor nomination at the 2009 SBS Drama Awards for the daily series Innocent You. In 2010, he was once again nominated for Best Supporting Actor at the SBS Drama Awards for the Joseon-era medical drama Jejoongwon. In 2018, he joined the MBC series Wealthy Son, which earned him a Best Supporting Actor nomination at the MBC Drama Awards.

Kang Nam-gil and his wife divorced in 2000 on the grounds of adultery on the latter’s side. He then moved to England with his son and daughter. They returned to South Korea after almost four years. In 2004, he published a book documenting his life in the foreign country.