In his youth, Nam Su-Il was active in running, gymnastics, soccer, and swimming, but it was weightlifting that would come to define his athletic life. He took up the sport in 1930 and eventually settled on the featherweight class, setting three world records in that discipline – two in the press and one in overall – between 1938 and 1940. During this time he was forced to compete under the Juitsu Nan for Japan due to the Japanese occupation for Korea. His greatest success in international tournaments came at the 1947 World Championships, where he won a silver medal behind Robert Higgins of the United States. Higgins was absent from the 1948 Summer Olympics, making Nam a strong contender for the Olympic title, but he ended up placing fourth at the Games. He continued to compete, and even made the Korean team for the 1952 Summer Olympics, but was past his peak by this point and finished 11th in a field of 22 competitors. Following his retirement from active competition, he worked as a weightlifting coach and served as the Executive Director of the South Korean Weightlifting Federation.