George Henry Thomas, United States Army officer, was born in Southampton County, Virginia, on July 31, 1816, the son of John and Elizabeth (Rochelle) Thomas. He entered the United States Military Academy at West Point on July 1, 1836, and graduated twelfth in the class of 1840. Among his classmates were Richard S. Ewell and William T. Sherman. Thomas was commissioned a second lieutenant in the Third Artillery on July 1. He won his first brevet, to first lieutenant, "for gallant and good conduct" in the Second Seminole War in Florida on November 6, 1841. He subsequently did garrison duty in a number of southern forts and was promoted to first lieutenant on April 30, 1844. In the Mexican War Thomas served in Braxton Bragg's field artillery in the army of Gen. Zachary Taylor; he was brevetted captain on September 23, 1846, for "gallant conduct" at the storming of Monterrey and major after the battle of Buena Vista on February 23, 1847. At the end of the war Thomas was again posted to Florida to fight Indians. From 1851 to 1854 he served as an instructor of artillery at West Point, where he received a promotion to captain on December 24, 1853. He served a tour of duty at Fort Yuma, Arizona Territory, until May 12, 1855, when he was promoted to major and transferred to the elite Second United States Cavalry on the Texas frontier. There he commanded Fort Mason and its garrison, companies B, C, and G of the regiment. Company B was commanded by Capt. Edmund Kirby Smith, and John Bell Hood was at the time a second lieutenant in Company G. When the Second Cavalry's second commanding officer, Col. Robert E. Lee, was granted an extended leave of absence on October 24, 1857, Thomas was given command of the regiment, a command he held until November 12, 1860. At the head of the military escort that removed the Indians from the Comanche and Brazos Agency reservations, Thomas conducted them and their agent, Robert S. Neighbors, from Fort Belknap to the Red River, across into Indian Territory, and to Fort Cobb; the trip took from August 1 to 16, 1859. During October and November of that year Thomas led the Cimarron expedition, a reconnaissance patrol from Fort Cooper to the headwaters of the Canadian and Red rivers and into New Mexico Territory that did not succeed in bringing the raiding Comanches to bay. In the summer of 1860 he led a similar scouting expedition to the headwaters of the Concho and Colorado rivers. On this expedition, on the Salt Fork of the Brazos on August 25, he received two superficial but painful arrow wounds to his chin and chest in an encounter with a elderly Comanche warrior, too old to keep up with retreating comrades. These were the only wounds this soldier of four wars ever received. On November 1, 1860, Thomas was granted a twelve-month leave of absence; he was in New York when Texas left the Union. After secession the Second Cavalry was withdrawn to Carlisle Barracks, Pennsylvania, where Thomas rejoined it on April 14.
When the Civil War broke out, Thomas, although a southerner by birth, chose to remain with the old army. He was promoted to lieutenant colonel on April 25, 1861, and to colonel on May 3 of the same year. After commanding a brigade in the Shenandoah valley, he was transferred to the Third Cavalry on August 3, 1861, the same day he received an appointment as brigadier general of volunteers. After being reassigned to the Army of the Ohio, Thomas led a division at the battle of Shiloh. He was promoted to major general of volunteers on April 25, 1862, and took part in the capture of Corinth, Mississippi, and Don Carlos Buell's campaign in Kentucky, which included the battle of Perryville in October 1862. As commander of the Fourteenth Corps of William S. Rosecrans's Army of the Cumberland, Thomas served at the battle of Murfreesboro (Stones River) on December 31, 1862-January 1, 1863, and won lasting fame as "the Rock of Chickamauga" on September 20, 1863, when he saved Rosecrans's army from disaster by his determined stand on Snodgrass Hill in the face of fierce Confederate assaults. This service won for Thomas a promotion to brigadier general in the regular army on October 27, 1863. He superseded Rosecrans as commander of the Army of the Cumberland and conducted a dogged defense of Chattanooga against Braxton Bragg's besieging Confederate army until he was reinforced by troops under Ulysses S. Grant and William T. Sherman. Breaking out of Chattanooga, Thomas won a decisive victory at Lookout Mountain, Tennessee, on November 25, thus paving the way for the Union offensive against Atlanta. As part of Sherman's army group, Thomas took part in the campaign in Georgia and the battle of Atlanta, where he faced his old Second Cavalry subordinate, John Bell Hood. When Hood unexpectedly marched into Tennessee after the fall of Atlanta, Thomas's army was detached from Sherman's command to follow him and bring him to battle. Thomas crushed the Confederate Army of Tennessee in two days of fighting at Nashville on December 15–16, 1864. On March 3, 1865, he received a vote of thanks from the Congress for his "skill and dauntless courage, by which the rebel army under Gen. Hood was signally defeated and driven from the State of Tennessee."
Thomas remained in command of the area until the end of the war, after which he declined promotions to lieutenant general and to full general and the nomination of the Republican party to the presidency. In June 1869 he was assigned to command of the Military Division of the Pacific with headquarters at San Francisco, where he died of apoplexy on March 28, 1870. Thomas was survived by his wife, the former Frances Lucretia Kellogg, to whom he had been married on November 17, 1852. The couple was childless. Thomas was buried at Troy, New York, his wife's hometown.