Thomas Green, military leader, was born in Buckingham County, Virginia, on June 8, 1814, to Nathan and Mary (Field) Green. The family moved to Tennessee in 1817. Green attended Jackson College in Tennessee and Princeton College in Kentucky before he received a degree from the University of Tennessee in 1834. He then studied law with his father, a prominent judge on the Tennessee Supreme Court. When the Texas Revolution began, he left Tennessee to join the volunteers. He reached Nacogdoches by December 1835 and enrolled for military service on January 14, 1836. He became one of Isaac N. Moreland's company, which operated the Twin Sisters cannons in the battle of San Jacinto on April 21, 1836. A few days after the battle Green was commissioned a lieutenant; in early May he was made a major and aide-de-camp to Thomas J. Rusk. He resigned on May 30 to continue studying law in Tennessee.
When he returned and settled in Texas in 1837, he was granted land in reward for his army service and became a county surveyor at La Grange, Fayette County. After his nomination by fellow San Jacinto veteran William W. Gant, he was elected engrossing clerk for the House of Representatives of the Republic of Texas, a post he held until 1839, when he represented Fayette County in the House of the Fourth Congress. After a term he chose not to run again and resumed the office of engrossing clerk. During the Sixth and Eighth congresses he served as secretary of the Senate. From 1841 to 1861 he was clerk of the state Supreme Court.
Between legislative and court sessions Green served in military campaigns against the Indians and Mexico. In late 1841 he joined Capt. Mark Lewis in a foray up the Colorado River against the Comanches. After Rafael Vásquez's invasion of San Antonio in March 1842, Green recruited and served as captain of the Travis County Volunteers, a unit that did not see battle. That fall he served as inspector general for the Somervell expedition after Adrián Woll's foray into San Antonio.
When the United States went to war with Mexico, Green recruited and commanded a company of Texas Rangers in La Grange as part of the First Texas Regiment of Mounted Riflemen, led by John C. Hays. The Texans helped Zachary Taylor capture Monterrey, Nuevo León, in September 1846. After returning home, Green married Mary Wallace Chalmers, daughter of John G. Chalmers, on January 31, 1847. Five daughters and one son were born to them.
After secession in 1861, Green was elected colonel of the Fifth Texas Volunteer Cavalry, which, as part of a brigade led by Gen. H. H. Sibley, joined the invasion of New Mexico in 1862. There Green led the Confederate victory at the battle of Valverde in February. After a difficult retreat into Texas he led his men, aboard the river steamer Bayou City, to assist in the recapture of Galveston on January 1, 1863. In the spring of 1863 Green commanded the First Cavalry Brigade in fighting along Bayou Teche in Louisiana. On May 20 he became a brigadier general. In June he captured a Union garrison at Brashear City but failed to seize Fort Butler on the Mississippi. At Cox's Plantation he defeated a Union advance in July. In September the First Cavalry captured another Union detachment at Stirling's Plantation. A similar success followed in November at Bayou Burbeaux. In four victories Green's men inflicted about 3,000 casualties and suffered only 600. In April 1864 he led a division in successful attacks against Maj. Gen. Nathaniel P. Banks at the battle of Mansfield and against Maj. Gen. William H. Emory at the battle of Pleasant Hill (see RED RIVER CAMPAIGN). A few days later, on April 12, 1864, Green died while leading an attack on federal gunboats patrolling the Red River at Blair's Landing. He was buried in the family plot at Oakwood Cemetery in Austin. Tom Green County was named for him in 1874.