The Pony Express was an American express mail service that used relays of horse-mounted riders between Missouri and California. It was operated by the Central Overland California and Pikes Peak Express Company.

During its 18 months of operation beginning in 1860, the Pony Express reduced the time for messages to travel between the east and west US coast to about 10 days. It became the west's most direct means of east–west communication before the first transcontinental telegraph was established (October 24, 1861), and was vital for tying the new state of California with the rest of the United States.

Despite a heavy subsidy, the Pony Express was not a financial success and went bankrupt in 18 months, when a faster telegraph service was established. Nevertheless, it demonstrated that a unified transcontinental system of communications could be established and operated year-round. When it was replaced by the telegraph, the Pony Express quickly became romanticized and became part of the lore of the Old West. Its reliance on the ability and endurance of hardy riders and fast horses was seen as evidence of rugged American individualism of the frontier times.

The idea of having a fast mail route to the Pacific Coast was prompted largely by California's newfound prominence and its rapidly growing population. After gold was discovered there in 1848, thousands of prospectors, investors, and businessmen made their way to California, at that time a new territory of the U.S. By 1850, California had entered the Union as a free state. By 1860, the population had grown to 380,000. The prospect of California and its national role became the source of bitter partisan debate in Congress. The demand for a faster way to get the mail and other communications to and from this westernmost state became even greater as the American Civil War approached.

William Russell, Alexander Majors, and William Bradford Waddell were the three founders of the Pony Express. They were already in the freighting and drayage business. At the peak of the operations, they employed 6,000 men, owned 75,000 oxen, thousands of wagons, and warehouses, plus a sawmill, a meatpacking plant, a bank, and an insurance company.

Russell was a prominent businessman, well respected among his peers and the community. Waddell was co-owner of the firm Morehead, Waddell & Co. In 1859, C. R. Morehead took the proposal for the Pony Express to President Buchanan. After Morehead was bought out and moved to Leavenworth to enter the mercantile business, Waddell merged his company with Russell's, changing the name to Waddell & Russell. In 1855, they took on a new partner, Alexander Majors, and founded the company of Russell, Majors & Waddell. They held government contracts for delivering army supplies to the western frontier, and Russell had a similar idea for contracts with the U.S. government for fast mail delivery.

By using a short route and mounted riders rather than traditional stagecoaches, they proposed to establish a fast mail service between St. Joseph, Missouri, and Sacramento, California, with letters delivered in 10 days, which many said was impossible. The initial price was set at $5 per 1⁄2 ounce (14 g), then $2.50, and by July 1861 to $1. The initial price was 250 times the price of mail through the normal mail service, which was $0.02. The founders of the Pony Express hoped to win an exclusive government mail contract, but that did not come about.

Russell, Majors, and Waddell organized and put together the Pony Express in two months in the winter of 1860. The undertaking assembled 80 riders, 184 stations, 400 horses, and several hundred personnel during January and February 1861.