In 1819, West Tennessee was divided into Surveyor Districts, with what would become Weakley County falling into parts of the 12th and 13th Surveyor's Districts. Soon after, the land was surveyed and divided, and then began to be distributed. On October 21, 1823, Tennessee's 15th General Assembly officially established Weakley County, naming it in honor of Robert Weakley (1764-1845). A transcription of the Act is found here.
By 1836, the county had been divided into twelve civil districts. A map of these civil districts can been seen here (courtesy of Tennessee State Library and Archives). A transcription of the description of each civil district, which accompanies the map, is here.
Originally rectangular in shape and containing over 700 square miles, some of Weakley County's land was later transferred to Gibson County to the south, and Obion County to the west, resulting in the current shape and size of the county, which now encompasses 576 square miles.