The history of vigilante justice and the Montana Vigilantes began in 1863 in what was at the time a remote part of eastern Idaho Territory. Vigilante activities continued, although somewhat sporadically, through the Montana Territorial period until the territory became the state of Montana on November 8, 1889. Vigilantism arose because territorial law enforcement and the courts had very little power in the remote mining camps during the territorial period.

Henry Plummer was a prospector, lawman, and outlaw in the American West in the 1850s and 1860s, who was known to have killed several men. He was elected sheriff of what was then Bannack, Idaho Territory, in 1863 and served until 1864, during which period he was accused of being the leader of a "road agent" gang of outlaws known as the "Innocents," who preyed on shipments from what was then Virginia City, Idaho Territory to other areas. In response some leaders in Virginia City formed the Vigilance Committee of Alder Gulch and began to take action against Plummer's gang, gaining confessions from a couple of men they arrested in early January 1864. On January 10, 1864, Plummer and two associates were arrested in Bannack by a company of the Vigilantes and summarily hanged. Plummer was given a posthumous trial in 1993 which led to a mistrial. The jury was split 6–6.