The National League in its current format was created in 2003 as the K2 League, but the N-League organisation has been running second-tier competitions since the 2000 season. Eleven sides competed for the first league tournament organised by the K2 in the spring of 2000, and Korea National Railroad finished top of the pack to lift the trophy. Two other cup competitions followed the spring league season which were won by Gangneung City Hall and Ulsan Mipo Dockyard respectively.
The spring league season of 2001 was won by Kookmin Bank and the military side Sangmu won the same competition in 2002 before the K2 League was reorganised into a two-stage league competition beneath the level of the K-League for 2003. Goyang Kookmin Bank, the reincarnation of the side that had competed as founder members of the K-League, would become the dominant K2 force, lifting both the 2003 and 2004 championships. Goyang's reign as champions was ended in 2005 as Incheon Korail lifted the crown, defeating Suwon City 4-2 in the playoffs to claim the title.
The league was rebranded the National League at the beginning of 2006, with Goyang Kookmin Bank reclaiming their crown in the end of season playoff, and they were replaced as champions by Ulsan Mipo Dockyard in 2007.