Thomas McKinney Jack, lawyer, soldier, and congressman, the son of Laura (Harrison) and William Houston Jack, was born at San Felipe de Austin, Texas, on December 19, 1831. He attended Georgetown College in Kentucky for a short time before graduating with honors from Yale University in 1853. He then returned to Texas and studied law in the Galveston office of his brother-in-law, William Pitt Ballinger. Jack was admitted to the bar at Galveston in 1855 in partnership with Ballinger. In 1856 he was elected probate judge (chief justice, according to some sources) of Galveston County, and in 1858 was elected to the House of Representatives after the resignation of John Henry Brown. In 1857 Jack married Nannie Mills Knox, a native of Lincoln County, Missouri, and a ward of her uncle Robert Mills in Galveston. They had four children.
In 1860 Jack, an ardent states'-rights Democrat, was a presidential elector on the John C. Brackenridge ticket, and when Texas seceded the next year he volunteered as an aid to Col. Sidney Sherman, who was organizing the defenses of Galveston. Jack then took part in the expedition to capture Fort Brown on the Rio Grande and returned to Galveston in time to volunteer as a private in Capt. John A. Wharton's company of the Eighth Texas Cavalry-better known as Terry's Texas Rangers. In December 1861, however, he was promoted to first lieutenant and transferred to the staff of Gen. Albert Sidney Johnston as aide-de-camp. When Johnston was mortally wounded at the battle of Shiloh in April 1862 he fell into Jack's arms and died. Jack was thereupon promoted to major and named adjutant general of Lt. Gen. Leonidas Polk's corps of the Army of Tennessee. In that capacity he fought at the battles of Perryville, Murfreesboro, and Chickamauga and was promoted to lieutenant colonel before Polk, too, was killed beside Jack at the battle of Pine Mountain, Georgia, on June 14, 1864. Jack was then promoted to colonel and transferred to the Trans-Mississippi Department, where he was named adjutant general of Texas. With his uncle, Brig. Gen. James E. Harrison, Jack was a member of the Confederate delegation that surrendered Texas to federal forces at Galveston at the end of the war.
He returned to an extensive and highly profitable legal practice and in July 1880 was a delegate to the national Democratic convention in Cincinnati. Soon after returning to Galveston he died of pneumonia, on August 26, 1880. Norman Goree Kittrell referred to Jack as "the most gracious, graceful, charming and courtly gentleman, and the most accomplished nisi prius lawyer ever at the Bar of Texas." Nannie Jack died in Galveston on January 12, 1881.