James Butler Hickok was born May 27, 1837, in Homer, Illinois, to William Alonzo Hickok, a farmer and abolitionist who used the family home as a station on the Underground Railroad. In 1855, at age 18, James fled Illinois following a fight with Charles Hudson, during which both fell into a canal, and each thought, mistakenly, that he had killed the other. Hickok moved to the Kansas Territory and began using his late father's name, William Hickok.

Photographs of Hickok appear to show dark hair, but all contemporaneous descriptions affirm that it was red.

In Kansas, Hickok joined an antislavery vigilante group known as the Jayhawkers. While a Jayhawker, he met 12-year-old William Cody (later known as "Buffalo Bill"), who, despite his youth, served as a scout just two years later for the U.S. Army.

By the summer of 1861, he was working as a stock tender at a stage depot in Nebraska called Rock Creek Station. Across the creek lived David McCanles, a mean-spirited man who disliked Hickok and called him "Duck Bill" for his long nose and protruding lips. Hickok took his revenge by secretly romancing McCanles' mistress, Sarah Shull. When McCanles learned about the affair, he threatened to drag "Duck Bill" outside and give him a thrashing. Demonstrating remarkable coolness for a 24-year-old who had never been involved in a gunfight, Hickok replied, "There will be one less son-of-a-bitch when you try that." McCanles ignored the warning, and Hickok shot him in the chest.

In 1860, Hickok was badly injured while driving a freight team from Independence, Missouri, to Santa Fe, New Mexico. According to Hickok's account, he found the road blocked by a cinnamon bear and its two cubs. Dismounting, he approached the bear and fired a shot into its head, but the bullet ricocheted off its skull, and the bear attacked, crushing Hickok with its body and grabbing his arm in its mouth. Hickok was able to grab his knife and slash the bear's throat, killing it, but he was bedridden for four months before being sent to Rock Creek Station in the Nebraska Territory to work as a stable hand while he recovered.

He fought and spied for the Union Army during the American Civil War, but in September 1862, he was discharged for unknown reasons. He gained publicity after the war as a scout, marksman, actor, and professional gambler.

While in Springfield, Hickok and a local gambler named Davis Tutt had several disagreements over unpaid gambling debts and their mutual affection for the same women. There were reports that Hickok had fathered an illegitimate child with Tutt's sister, while Tutt had been observed paying a great deal of attention to Wild Bill's paramour, Susanna Moore. On July 21, 1865, the two men faced off in Springfield's town square, standing sideways before drawing and firing their weapons. It is one of the few recorded instances in the Old West of a one-on-one pistol quick-draw duel in a public place, in the manner later made iconic by countless dime novels, radio dramas, and Western films such as High Noon. Two days later, Hickok was arrested for murder, but the charge was later reduced to manslaughter.

Wild Bill served as a scout for General George Armstrong Custer's 7th Cavalry during the Indian Wars. Custer's wife, Libbie, was dazzled by Hickok, and there were rumors of an affair. In her 1890 book Following the Guidon, Libbie wrote: "Physically, he was a delight to look upon. Tall, lithe, and free in every motion, he rode and walked as if every muscle was perfection, and the careless swing of his body as he moved seemed perfectly in keeping with the man, the country, the time in which he lived. I do not recall anything finer in the way of physical perfection than Wild Bill when he swung himself lightly from his saddle, and with graceful, swaying step, squarely set shoulders and well poised head, approached our tent for orders."

On April 15, 1871, Hickok became marshal of Abilene, Kansas, replacing Tom "Bear River" Smith, who had been killed while serving an arrest warrant. Outlaw John Wesley Hardin arrived in Abilene at the end of a cattle drive shortly thereafter. Hardin was a well-known gunfighter, and is known to have killed more than 27 men. In his 1895 autobiography, published after his death, Hardin claimed to have been befriended by Hickok, the newly elected town marshal, after he had disarmed the marshal using the road agent's spin. Hickok later said he did not know that "Wesley Clemmons" was Hardin's alias, and that he was a wanted outlaw. In August 1871, Hickok sought to arrest Hardin for killing Charles Couger in an Abilene hotel "for snoring too loud," but Hardin left Kansas before Hickok could arrest him.

According to Joseph Rosa, Hickok's biographer and the foremost authority on Wild Bill, he killed six or possibly seven--a far cry from the "hundreds" of bad hombres he claimed to have slain.