In the aftermath of World War I, the part of Nógrád county north of the river Ipeľ/Ipoly became part of newly formed Czechoslovakia (as Novohrad county), as recognized by the concerned states in 1920 by the Treaty of Trianon.
The Hungarian part of Nógrád merged with the Hungarian part of Hont county to form Nógrád-Hont county.