When Montana was made a Territory, May 26, 1864, the pioneers began to plan for a legislative session, in accordance with the act creating the territory. Sidney Edgerton was the first governor of Montana, being appointed by President Lincoln while he was on the way home from Washington, D. C., where he was one of a group securing the consent of the president, and assisting in arranging the details necessary to make Montana a territory. Members of a territorial legislature were chosen and convened at the town of Bannack, Montana Territory, December 12, 1864.

An act was approved by this legislative assembly on February 2, 1865, creating the counties of Beaverhead, Big Horn, Chouteau, Deer Lodge, Gallatin, Jefferson, Edgerton (later becoming Lewis and Clark), Madison and Missoula.

All but Big Horn were authorized to perfect their organizations, but Big Horn was attached to Gallatin County for judicial and legislative purposes, as there were few white men in the county. At that time, Big Horn County, which included that entire portion of the territory not included in the boundaries named for the other eight counties, contained practically 56,284 square miles. This, with Gallatin’s official quota made an area of nearly 80,000 square miles, more than half the territory within the limits of Montana. For many years, Gallatin carried this burden until Big Horn County passed out of existence and other counties were formed from this area. Some of the counties formed or at least partially formed from Big Horn County are: Park, Sweetgrass, Stillwater, Yellowstone, Carbon, Custer, Dawson and Wheatland.