The San Pedro, Los Angeles & Salt Lake Railroad, the first direct route from Salt Lake City to Los Angeles when it was completed in 1905, was perhaps the single most significant factor in the creation of what would become the city of Las Vegas, and later, Clark County.

Starting in the mid-1850s, the Las Vegas Valley had been an oasis for travelers on the Mormon Trail, thanks to the area's supply of water and relatively fertile ground for growing crops. Few people lived in the valley during the second half of the nineteenth century, notably pioneer ranchers Archibald Stewart, his wife, Helen J. Stewart, their ranch hands, and some members of the Southern Paiute tribe. By 1900, only about thirty people lived in the Las Vegas Valley.

By the end of the nineteenth century, there was still no railroad linking the Great Basin's largest community, Salt Lake City, to the biggest metropolitan area in the southwestern United States, Los Angeles. Competition to build such a rail link pitted U.S. Senator William A. Clark of Montana against Edward H. Harriman, the owner of the Union Pacific. Clark wanted to send ore from his copper mines west, and Harriman sought to expand its already established railroad.

Harriman's Union Pacific made the first move in 1899, sending a series of short rail lines in Utah to Cedar City, near the Utah border with Nevada—about seventy miles east of the Las Vegas Valley. Clark entered the fray in 1900 by buying a small rail line, the Los Angeles Terminal Railroad. He renamed it the San Pedro, Los Angeles & Salt Lake Railroad.

Clark wanted to beat the Union Pacific's train, the Oregon Short Line, in the race to send a railroad through southern Nevada to Los Angeles. Both Clark and Union Pacific—each laying claim in court to holding the rights-of-way for their rail lines—began constructing grades for their rails in 1901. The rivalry resulted in at least one violent fight among the workers.

Union Pacific first obtained an option to buy ranch land and water rights in the Las Vegas Valley in 1901, but it failed to complete the purchase. In 1902, Clark bought the Stewart Ranch from Archibald Stewart's widow, Helen. Clark acquired the 1,800 acres for $55,000, with only $5,000 as a down payment. In addition, Clark got the Stewart ranch house and the Las Vegas Springs, the ranch's water source. He also purchased the nearby Kiel Ranch.

But the competition between Clark and Harriman continued. Both started building rail lines westward from Utah. The crews of the two rail builders met just north of Las Vegas at the Meadow Valley Ranch. In what would be called the Clark-Harriman War, both men claimed the right to build into the valley. The two rail barons finally negotiated a compromise. Clark was permitted to operate the San Pedro, Los Angeles & Salt Lake on the tracks in exchange for giving Union Pacific a fifty percent share in the railroad.

The San Pedro, Los Angeles & Salt Lake was completed in Jean, Nevada, twenty miles west of Las Vegas, on January 30, 1905. Trains started running through Las Vegas a few days later, although regular service did not start until May 1905.

Clark intended to transform his land in Las Vegas into a major train station with a staging area for his main railroad and other local short lines, as well as a place to store train equipment. He also wanted to sell property in small lots so that a town could be built there, including housing for his employees.

On May 15, 1905, Clark's executives held a two-day auction for commercial and residential lots in the area he called the Clark Las Vegas Townsite—now Main Street just north of Fremont Street. Dozens of prospective buyers took the train from Los Angeles and elsewhere to attend the auction. Although the nearby McWilliams Townsite had been established the previous year, most historians consider Clark's auction of lots the beginning of Las Vegas as a town.

Clark's railroad, which helped create other towns along its route, reduced the time it took to travel from Salt Lake City to Los Angeles to a single day. He soon completed a short line, the Las Vegas & Tonopah Railroad, that carried freight from Las Vegas 120 miles north to Beatty, Nevada, near the Bullfrog district. Some mistakenly believed that the area had rich mining potential.

By 1909, Clark enlarged the Las Vegas train yard and repair shop, and was employing 400 people. That year, legislators voted to create a new county and named it for him. He built a series of cottages for his workers, from 2nd to 4th streets downtown. In 1911, of the town's population of fifteen hundred, 450 residents worked for Clark's railroad.

Due to economic hard times, the railroad halted some services in Las Vegas in 1917. Clark's line to Beatty, which was losing money, was discontinued in 1918, and its rails and ties were removed. Finally, in 1922, Clark sold his share in the railroad to Union Pacific.