It was not until 1843 that the first intrepid settlers came north and established the Doņa Ana Bend Colony, the county's first permanent settlement. In 1846 New Mexico became part of the United States as a result of the U.S.-Mexican War. By 1855 Doņa Ana County included the entire southern portion of the territory extending from Texas on the east and south, to the Colorado River on the west, and from the Mexican border on the south to Socorro County on the north.

As New Mexico grew in population, Arizona and new counties were carved from the original boundaries until Doņa Ana County became its present size and communities such as Las Cruces, Mesilla, and Fort Selden, full of years of history and memories, produced a legacy worthy of preservation.