The Fourth Texas Cavalry, also known as Fourth Regiment, Texas Mounted Volunteers, was organized in September 1861 and was assigned to Henry H. Sibley's Army of New Mexico. The men were primarily recruited from the counties of Caldwell, Cherokee, Dewitt, Guadalupe, Houston, Milam, Nacogdoches, and Victoria. On August 20, 1861, Sibley appointed James Reily commander of "Reily's Regiment" as colonel. Reily never exercised command, however, being detached, instead, on a diplomatic mission to the governors of Chihuahua and Senora. He was replaced as head of the regiment by his lieutenant colonel, William Read "Dirty Shirt" Scurry whose commission, also issued by Sibley, dated August 23, 1861. William Polk "Gotch" Hardeman, the original captain of Company A, became the regiment's third commanding officer on March 28, 1862.
During the Sibley campaign, the regiment played a conspicuous role in the battles of Valverde, the siege of Albuquerque, and the disastrous battle of Glorieta. Of the 810 effectives who marched out of San Antonio on October 10, 1861, for Fort Bliss and the invasion of New Mexico Territory, 47 were killed or mortally wounded in battle, 28 died of disease, 92 were wounded, and 194 were taken prisoner.
Following the retreat to San Antonio, the regiment was reassigned to Tom Green's cavalry brigade in which it saw service at the recapture of Galveston on January 1, 1863, (see GALVESTON, BATTLE OF). For the remainder of 1863, the regiment served in the defense of south Louisiana, including action at Fort Bisland, April 12–13; Irish Bend, April 14; Brashear City, June 23; Donaldsonville, June 28, 1863; Cox's Plantation, July 12–13; Stirling' Plantation, September 29; and Bayou Bourbeau on November 3.
In March 1864 it became part of Maj. Gen. Richard Taylor's army opposing Maj. Gen. Nathaniel P. Banks's Red River campaign and was heavily engaged at Mansfield on April 8, 1864, and Pleasant Hill on April 9, 1864. The regiment surrendered with Lt. Gen. Edmund Kirby Smith at Shreveport, Louisiana, on May 26, 1865.