The first nine counties in the territory to be created, in 1852, were Bernalillo, Doña Ana, Rio Arriba, San Miguel, Santa Ana, Santa Fe, Socorro, Taos, and Valencia Counties.

Following the Gadsden Purchase of 1853–1854, the northeasternmost part of the New Mexico Territory was ceded to the new Colorado Territory in February 1861, before the western half was reorganized as the Arizona Territory in February 1863, establishing New Mexico's present-day boundaries.

New Mexico was admitted to the Union as the 47th state on January 6, 1912.

The Federal Information Processing Standard (FIPS) code, which is used by the United States government to uniquely identify states and counties, is provided with each entry.

For comparison, the population estimate for the state of New Mexico as of July 2011 was 2,082,224, and the area was 121,589 mi2 (315,194 km2).