Old Miller County was created April 1, 1820 by the Arkansas Territorial Legislature

Arkansas Territory

The Territory of Arkansas, initially organized as the Territory of Arkansaw, was an organized incorporated territory of the United States that existed from July 4, 1819 until June 15, 1836, when the final extent of the territory was admitted to the Union as the State of Arkansas.-History:The...

. It included much of what is southeastern Oklahoma

Little Dixie (Oklahoma)

Little Dixie is the name given to the southeast region of Oklahoma, which is heavily influenced by southern "Dixie" culture, as it was settled chiefly by Southerners seeking a start in new lands following the American Civil War...

 and the northeastern counties in Texas (Bowie, Red River, Lamar, Fannin, Cass, Morris, Titus, Franklin, Hopkins, Delta and Hunt).

The Act delineated Miller County as follows:

Be it enacted by the General Assembly of the Territory of Arkansas, That all that portion of the County of Hempstead and bounded as follows, to-wit: Beginning on the north bank of the great Red River, at a point due south of the Cossetat Bayou, a branch of Little River, thence due north to the mouth of the Cossetat Bayou aforesaid, then up said bayou to the head of its main branch, then north to the boundary line of Clark, then due west with said line to the Canadian river, or the Indian boundary line, then with the said line to the great Red river aforesaid, then southeasterly with the Indian or Spanish boundary line to a point due south of the point of beginning, then due north to the beginning, to be laid off and erected into a separate county, to be called and known by the name of the county of Miller.

The 1820 territorial census listed 999 residents in Miller County, 82 of them slaves. In August 1821, the county commissioners selected Miller Court House as the county seat. The exact location is now unknown except that it was in what is currently McCurtain County, Oklahoma. The post office

Miller Court House

Miller Court House, was the first post office located in what is now Oklahoma, United States. It was located in what was then Miller County, Arkansas Territory. The post office opened September 5, 1824, and was closed December 28, 1839. Miller Court House was the county seat of old Miller County...

 was established September 7, 1824.

Although settlers continued to move into the area, the Treaty of Doak's Stand

Treaty of Doak's Stand

The Treaty of Doak's Stand was signed on October 18, 1820 between the United States and the Choctaw Indian tribe. Based on the terms of the accord, the Choctaw agreed to give up approximately one-half of their remaining Choctaw homeland...

(October 18, 1820) was about to change Miller County. After Doak's Stand, Choctaws had already been moving into the area of Arkansas Territory, but a treaty signed January 20, 1825, ceded the land west of a line "one hundred paces east of Fort Smith, and running thence, due south, to Red river" to them in exchange for their land in the East. The resident of Miller County signed petitions, the territorial government pressured Washington, but all to no avail. Finally, on October 17, 1828, the territorial legislature abolished Miller County north of the Red River and added the remnant of that county east of the new boundary line to Sevier County. The remaining residents burned the courthouse at Miller Court House and all the records.

The commissioners moved the county seat to Jonesborough plantation south of the Red River on October 23, 1832, and the Miller Court House post office relocated there. However, Miller County south of the Red River was in dispute with the Mexican government. After Texas seceded, they attempted to enforce their claims to the area. In 1838, Texas formed Fannin County, and Washington finally discontinued the Miller Court House post office on December 28, 1838. When Texas joined the Union in 1845 the borders became permanent.