On February 8, 1860, the Texas legislature passed an act providing for the formation of Greer County, with boundaries "beginning at the confluence of Red River and Prairie Dog Town River; then running up Red River, passing the mouth of the South Fork [Elm Fork] and following main or North Red River to its intersection with the twenty-third degree of west longitude (the 100th meridian); thence due south across the Salt Fork to Prairie Dog River, and thence following that river to the place of beginning." The act went into effect at once, but because of the confusion consequent to the outbreak of the Civil War little was done immediately toward organizing and putting into operation a system of county government. In July 1886 the settlers of Greer County met at Mobeetie and organized Greer County on the authority of the act of 1860. Mangum was named the county seat, and provision was made for a county government.
But the comparatively rapid development of Greer County was disturbed by a dispute between Texas and the United States over the ownership of the area. The controversy had its origin in the Adams-Onís Treaty of 1819, which designated the boundary between Spanish territory and the United States. The part of that treaty affecting Greer County provided that the boundary should follow "the course of Red River westward to the degree of longitude one hundred west from London and twenty-three from Washington; then crossing the said Red River and running thence by a line due north to the River Arkansas." Accompanying the treaty was the Melish map, on which the boundary line had been delineated. This map, as was later discovered, embodied two errors that were largely responsible for the dispute between Texas and the United States. According to this map the 100th meridian was from ninety to 100 miles farther east than the true 100th meridian; furthermore, the Red River, in its upper course, divides into two major branches instead of having only one as shown on the Melish Map. The controversy hinged about two points growing out of these errors. First, was the true 100th meridian or the 100th meridian shown on the Melish Map the boundary according to the makers of the treaty? Second, if the meridian accepted as the boundary proved to be west of the junction of the two forks of the Red River, which of these forks was the Red River of the treaty, and consequently the boundary? The United States took the position that the true 100th meridian was meant, and that the South Fork was the main Red River, and therefore that the boundary was along the South Fork of the Red River to the true 100th meridian and thence northward. The acceptance of this position would make Greer County a territory of the United States, since the true 100th meridian, when finally definitely located, was near the western boundary of Greer County, many miles west of the junction of the two forks of Red River. Texas held that the 100th meridian of the Melish map was the boundary intended by the framers of the treaty and that the North Fork was the main Red River and therefore the boundary. If Texas could successfully defend either of these contentions, she would be able to establish her claim to Greer County. Since the Melish 100th meridian was east of the junction of the North and South forks of the Red River, Greer County would be in Texas if that meridian were accepted as the boundary, for the disputed territory lay south and west of that stream.
These positions were argued and defined in an exchange of notes between the officials of the two governments over a period of years. In February 1886 the Texas Boundary Commission, meeting first in Galveston and later in Austin, sought a solution to the problem but accomplished little more than defining more fully and clearly the issues involved. In 1890 President W. H. Harrison approved an act providing for the organization of Oklahoma as a territory and the prosecution of a suit against Texas for a final settlement of the dispute over the ownership of Greer County. The suit was filed in 1891 by the attorney general of the United States. A demurrer of Texas that the Supreme Court had no jurisdiction in the case was overruled, and the original bill was taken up in October 1895. In setting forth its argument for the possession of the disputed territory, the United States laid down the following principles and claims on which the decision should be based: (1) The treaty of 1819 and the Melish map attached must be accepted as a basis of any conclusion reached. (2) In the light of the language of the entire treaty the framers of that instrument intended that the true 100th meridian rather than the 100th meridian of the Melish map was the boundary line. (3) The Prairie Dog Town Fork of Red River is the continuation, going from east to west, of the Red River of the treaty, and the line, going from east to west, extends up Red River and along the Prairie Dog Town Fork to the 100th meridian and not up the North Fork of the Red River. (4) In conclusion, it was declared that Greer County was subject to the jurisdiction of the United States. The defense presented its argument under three major heads: (1) that the 100th meridian as delineated on the Melish map according to the intents and purposes of the framers of the treaty of 1819 was the real boundary between Texas and the United States; (2) that, if the true 100th meridian were accepted as the meridian of the Adams-Onís Treaty, and if that meridian, when located, were west of the confluence of the two forks, Greer County would still be within the boundary of Texas, since the North Fork was the Red River of the treaty rather than the South Fork, which was barely known to the Indians and early explorers because of its bad water; (3) and that the disputed territory belonged to Texas by right of possession and occupation, which possession extended back thirty-five years and had resulted in a well-ordered and prosperous land.
On the basis of these arguments and contentions, the Supreme Court, on March 16, 1896, delivered its opinion: (1) Taking the language of the treaty of 1819 as a whole, the true 100th meridian was meant as the boundary instead of that shown on the Melish map. (2) The South Fork of the Red River, considering its greater width and length, its delineation on many maps, and its continuation from east to west, was the boundary between the United States and Texas, rather than the North Fork. (3) The territory known as Greer County was not under the jurisdiction of Texas but under that of the United States. Greer County thus became a territory of the United States and, in 1906, part of Oklahoma. Although the decision of the Supreme Court had determined the ownership of Greer County, it did not at that time locate and mark the 100th meridian. The meridian that had been run in 1858 by A. H. Jones and H. M. C. Brown for the commissioner of Indian affairs had served in a general way as the western boundary of Greer County. Other persons, including H. S. Pritchett for the state of Texas in 1892 and Arthur Kidder for the United States in 1902, had located this meridian. The work of these men, however, had not for one reason or another proved satisfactory to all concerned and for that reason caused much dissatisfaction among those owning land along the meridian. Finally, in 1930, after Oklahoma had become a state and consequently was interested in a satisfactory location of the 100th meridian, which was a part of her western boundary, the Supreme Court, with Texas and Oklahoma concurring, accepted the surveying and marking of that meridian as done by Commissioner Samuel S. Gannett.
Then Greer County was part of Indian Territory, then Oklahoma. -Texas Handbook Online: Greer County