Just as the French had disputed Spain's claim to the area near the Red River, so also did American settlers. Claiming that the area was part of the Louisiana Purchase, American hunters and traders were active in the area by 1815. With the successful conclusion of the Texas Revolution, the United States relinquished its disputed claim to the area south of the Red River.
During the First Congress of the Republic of Texas, the Red River district was little more than a vaguely defined area encompassing all or part of the land of thirty-nine present counties. Red River County was formally delineated by an act signed by President Sam Houston on December 14, 1837, which divided the Red River District into two counties, Fannin and Red River. Red River County, as defined by the 1837 act, included all or part of lands now belonging to Lamar, Hopkins, Delta, Franklin, Titus, Morris, Cass, Marion, and Bowie counties. The act called for the first county courts to be held at LaGrange (later Madras) until a commission to be appointed by the county court could ascertain the proper location for a county seat. When the commissioners chose Clarksville in 1838, some residents of LaGrange tried unsuccessfully to reverse the ruling. New counties established in 1840 and 1846 reduced Red River County to its current size.
-Handbook of Texas Online: Red River County