Thomas B. McClendon was born around 1841 in Mississippi to William and Jemima McClendon. His father was a farmer who owned $6,400 of real estate and $11,800 of personal property in 1860. He grew up and attended school in Rankin County, Mississippi, until the family moved to Sumpter, Texas, sometime in the late 1850s. He enlisted in the Confederate army on August 31, 1861, and mustered in as a private in Company E of the 7th Texas Cavalry. He remained in the army until at least January 1864, and he eventually earned a promotion to sergeant.
He returned to Texas after leaving the army, and he married Margaret Hill in November 1865. They had at least three children: Frances, born around 1870; Frederick, born around 1874; James, born around 1880; and Mamie, born around 1886. In 1870, McClendon was working as a farmer in Pennington, Texas, and he owned $300 of personal property. His daughter Frances probably died sometime in the 1870s. By 1900, they were living in Justice, Texas. McClendon died on December 7, 1901.