Hickok used his late brother's name, William Hickok, from 1858, and the name William Haycock during the American Civil War. Most newspapers referred to him as William Haycock until 1869. He was arrested while using the name Haycock in 1865. He afterward resumed using his given name, James Hickok. Military records after 1865 list him as Hickok, but note that he was also known as Haycock. In an 1867 article about his shootout with Davis Tutt, his surname was misspelled as Hitchcock.
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. Hickok lost a gold watch to Tutt in a poker game. The watch had great sentimental value to Hickok, so he asked Tutt not to wear it in public. They initially agreed not to fight over the watch, but when Hickok saw Tutt wearing it, he warned him to stay away. On July 21, 1865, the two men faced off in Springfield's town square, standing sideways before drawing and firing their weapons. Their quick-draw duel was recorded as the first of its kind. Tutt's shot missed, but Hickok's struck Tutt through the heart from about 75 yards (69 m) away. Tutt called out, "Boys, I'm killed", before he collapsed and died.
Two days later, Hickok was arrested for murder. The charge was later reduced to manslaughter. He was released on $2,000 bail and stood trial on August 3, 1865. At the end of the trial, Judge Sempronius H. Boyd told the jury they could not find Hickok acted in self-defense if he could have reasonably avoided the fight. However, if they felt the threat of danger was real and imminent, he instructed they could apply the unwritten law of the "fair fight" and acquit. The jury voted to clear Hickok, resulting in public backlash and criticism of the verdict.
In September 1865, Hickok came in second in the election for city marshal of Springfield. Leaving Springfield, he was recommended for the position of deputy federal marshal at Fort Riley, Kansas. This was during the Indian Wars, in which Hickok sometimes served as a scout for General George A. Custer's 7th Cavalry.
In 1865, Hickok recruited six Native Americans and three cowboys to accompany him to Niagara Falls, where he put on an outdoor demonstration called The Daring Buffalo Chasers of the Plains. Since the event was outdoors, he could not compel people to pay, and the venture was a financial failure. The show featured six buffalo, a bear, and a monkey, and one show ended in disaster when a buffalo refused to act, prompting Hickok to fire a bullet into the sky. This angered the buffalo and panicked audience members, causing the animals to break free of their wire fencing and chase audience members, some of whom were trampled. The incident helped contribute to the overall failure of the show.