Gold was discovered near Pikes Peak in 1858, and the population of Colorado soared. Denver and other cities sprang up in this year. Fortune-seekers known as Fifty-Niners descended on the area by the thousands the following year. The Rocky Mountain region's first newspaper, the Rocky Mountain News, began in 1859 as well.

Congress created the Colorado Territory in 1861, about the same time that southern states were seceding from the Union. The creation of this territory rendered obsolete the federally unrecognized Territory of Jefferson, a self-proclaimed area of land that developed out of the Gold Rush and had its own legislature for a time. Growth surged again after the completion of the Denver Pacific Railway in 1870. The Kansas Pacific Railway reached Denver in that same year. Several other rail companies, including the Union Pacific line, had track in Colorado as well.

The Civil War had minimal effect on Colorado. Colorado volunteers took part in the Battle of Glorieta Pass, a strategic victory for the Union in what is now New Mexico.

As with other states, Colorado saw its share of land disputes and violent confrontations between Americans and Native Americans. Various treaties resulted in land cessions through the years. The Fort Laramie Treaty of 1851 had guaranteed Arapahoe and Cheyenne peoples ownership of Colorado land. The discovery of gold in 1858 changed that.

The U.S. Army established Camp Collins, near Colona, to protect the Overland Trail from Native American attacks. One particularly violent conflict was the Sand Creek Massacre, in 1864. A group of U.S. Army soldiers killed a few hundred Native Americans, mostly women and children, and then burned their village. This was part of a larger struggle known as the Colorado War.

More settlers came to Colorado as the 19th Century progressed. More explorers did, too. A group of men led by the famous John Wesley Powell was the first known to ascend 14,259-foot-tall Longs Peak, near what is now Estes Park.

It took Colorado residents a few attempts to gain statehood. They rejected the first state constitution drafted and then, after approving a second one, found the bill organizing their statehood approved by Congress but vetoed by President Andrew Johnson. Finally, on August 1, 1876, Colorado became the 38th state in the Union; the capital was Denver.