Thomas McKinney Jack was born on December 19, 1831, in San Felipe, Texas, to William Houston Jack and Laura Harrison. His father served as a congressman and secretary of state for the Republic of Congress, and he died of yellow fever on August 20, 1844. Jack graduated from Yale College in 1853 before moving to Galveston, Texas, to study law.
He was admitted to the bar in 1855, and he became a probate judge the following year. He married Nannie Mills around 1857, and they had at least five children: Louisa, born around 1857; David, born around 1859; Halley, born around 1865; William, born around 1868; and Thomas, born around 1872. He supported the Democratic Party, and local residents elected him to Congress in 1858. Two years later, he served as a presidential elector for Southern Democratic candidate John C. Breckinridge.
He enlisted in the Confederate army and became a private in the 8th Texas Cavalry. In December 1861, he was promoted to 1st lieutenant and appointed to the staff of General Albert Sidney Johnston. In the spring of 1862, he was promoted to major and adjutant general on the staff of General Leonidas Polk. He took part in the Battle of Perryville, the Battle of Murfreesboro, and the Battle of Chickamauga. He was promoted to colonel in 1864 and transferred to the Trans-Mississippi Department.
He returned to Galveston after the war and resumed his legal practice. By 1870, he owned $60,000 of real estate. He remained committed to the Democratic Party, serving as a delegate to the National Democratic Convention in 1880. He died of pneumonia in Galveston on August 26, 1880.