The land that later became New Mexico was patched together over 50 years. Part of the northeastern New Mexico was included in the Louisiana Purchase from France in 1803. The western portion of New Mexico came from the Treaty of Guadalupe Hidalgo in 1848, while the remainder of eastern New Mexico (from the present New Mexico-Texas border to the Pecos River) was added as consequence of the Compromise of 1850. The Gadsden Purchase of 1853 added further acreage to New Mexico Territory--the southernmost strip of Arizona and New Mexico.

The land contained in the original 1850 New Mexico Territory was the western portion of the future state, plus most of future Arizona (known as Santa Ana County), a small part of Colorado and Nevada south of 36[d]30[m]. The Texas addition and the Gadsen Purchase expanded the territory greatly, but the establishment of Colorado Territory in February 28, 1861 and Arizona Territory in February 24, 1863 (west of the 109th meridian), left New Mexico with its present boundaries.

As the route to California, New Mexico and Arizona were disputed territory during the American Civil War, resulting in an attempt to join the Confederate States of America and the Battle of Glorieta Pass.