Robert Leftwich, a representative for the Texas Association of Nashville, Tennessee, obtained a colonization grant from Mexico in 1825 that included the Milam County area. The grant's boundaries followed the Navasota River, turned southwest along the San Antonio road to the divide between the Brazos and the Colorado rivers, then northwest to the Comanche Trail, and east back to the Navasota.
It was during the first Congress of the Republic of Texas that the municipality came to be called Milam County. At that time the boundaries of the county were roughly the same as those of the colony granted to Leftwich, comprising one-sixth of the land area of Texas. In addition to the present Milam County, the counties of Bell, Bosque, Burleson, Coryell, Erath, Falls, Hamilton, Hood, Jones, McLennan, Robertson, Shackelford, Somerville, Stephens, and Williamson were eventually carved out of the original Milam County. Brazos, Brown, Burnet, Callahan, Comanche, Eastland, Haskell, Hill, Johnson, Lampasas, Lee, Limestone, Mills, Palo Pinto, Parker, Stonewall, Throckmorton, and Young counties also received land from Milam County. By 1850, with the exception of a small area between Williamson and Bell counties, Milam County had been reduced to its present size.
Present-day Milam County is in east central Texas 150 miles inland from the Gulf of Mexico and is bordered by Robertson, Burleson, Lee, Williamson, Bell, and Falls counties. Cameron, the county seat, is at the intersection of U.S. highways 77 and 190 on the Atchison, Topeka and Santa Fe Railway, sixty miles northeast of Austin and 140 miles south of Dallas. The county's center lies four miles south of Cameron at 30°47' north latitude and 96°59' west longitude. The present county covers 1,019 square miles of level to slightly rolling terrain at an elevation that ranges from 250 to 600 feet above sea level. The southern and eastern portions of the county lie in the post oak savannah region of the state, and the northern and western portions lie in the blackland prairie. The land is drained by the Brazos River, which forms the northeastern boundary of the county, by the Little River, which enters the county near the northwestern corner and winds to its mouth on the Brazos in the southeastern quadrant of the county, and by the San Gabriel River, which flows through the west central portion of the county to its mouth on the Little River.
Exactly which county begot and which begets is shown in the following table. Starting at the left with Milam County, each splitting off of other counties, or parts of counties, to the right.