Collin County was demarked from Fannin County on April 3, 1846, and named for Collin McKinney, one of the first settlers of the county and a signer of the Texas Declaration of Independence. The original county seat was Buckner. Because this town Buckner was not within three miles of the center of the county, however, McKinney became the county seat in 1848. Like the county, McKinney was named for Collin McKinney.
The settlement of Collin County can be divided into two phases. The first occurred during the early period of the county's history, from 1840 to 1860. The second phase took place during and after the arrival of railroads. The settlements established before the construction of rail lines seldom survived if the railroads bypassed them.
The influence of James W. Throckmorton, a native of McKinney and Texas state senator, resulted in Collin County's vote against secession, 948 to 405, in 1861. Once Texas joined the Confederacy, however, more than 1,500 residents of the county enlisted in the defense of the South, led by Throckmorton, who rose to the rank of brigadier general.