The Twenty-first Texas Cavalry served in the Trans-Mississippi Department as part of Parsons's Brigade for much of the Civil War. In the spring of 1862 Methodist minister George Washington Carter received permission from Richmond to recruit a regiment of lancers. Highly publicized in South Central Texas, the regiment was to be the only one of lancers in Confederate service. The idea of serving as a lancer was very popular among Texans facing the possibility of induction into the infantry, and Carter recruited enough companies to allow his lieutenant colonel and major to take command of separate regiments, with Carter becoming senior colonel. Carter's brigade of lancers originally included the First, Second, and Third Texas Lancers but were later designated the Twenty-first, Twenty-fourth, and Twenty-fifth Texas cavalry regiments, commanded by Carter (who led the Twenty-first), as senior colonel. The companies of the Twenty-first Texas came generally from the following counties: Company A from Walker; Company B from Burnet and Travis; Company C from Montgomery and Anderson; Company D from Bell and Milam; Company E from Austin and Lavaca; Company F from Washington; Company G from Orange; Company H from Grimes; Company I from Brazos; Company K from Goliad; and Company L from McLennan.
Soon after Carter’s brigade arrived in Arkansas, the bond between the three colonels reached a breaking point. The Twenty-fourth and Twenty-fifth were detached, and the men dismounted. Carter quickly aligned his regiment with the cavalry brigade of Col. William Henry Parsons to prevent the men in the Twenty-first from also losing their horses. The Twenty-first became regular cavalry (the lances had never arrived). Carter spent much time absent from the command, partly because of interest elsewhere and partly because he thought he should be the senior colonel of Parson's Brigade. Conflict between Carter and Parsons generally meant that the Twenty-first operated independent of Parsons. The Twenty-first Texas was led by Lt. Col. DeWitt Clinton Giddings in Carter's absence.
The Twenty-first participated in Brig. Gen. John S. Marmaduke's invasion of Missouri in the spring of 1863. During this raid on Cape Girardeau between April 17 and May 2, 1863, Carter commanded the "Texas Brigade," which included all the units in Parsons's Brigade except the Twelfth Texas. Upon returning to Arkansas, the Twenty-first remained in the southern part of the state. The troopers did not take part in the Confederate attempt to hold Little Rock, as they had been on picket duty along the Arkansas River when the state capital fell on September 10. After Pine Bluff also surrendered, the Twenty-first served as scouts and raiders in the region and often operated under the command of Maj. Benjamin D. Chenoweth.
The Twenty-first saw no major action until Maj. Gen. Nathaniel Banks's invasion of Louisiana in 1864. The regiment did not arrive in time to take part in the fighting at Mansfield and Pleasant Hill but did participate in following the Federal army on its retreat down the Red River and fought numerous skirmishes with Union cavalry and infantry. The Twenty-first saw its last action at the battle of Yellow Bayou on May 18, 1864.
Returning to Arkansas, the Twenty-first again operated as scouts and raiders. Problems of command between Parsons and Carter continued, so early in 1865 authorities transferred the Twenty-first to a brigade under Walter P. Lane. The regiment was in Texas when it disbanded in the spring of 1865. Throughout the war the Twenty-first had taken part in two important events: Marmaduke's invasion of Missouri and the Red River campaign. Their service as scout and raiders helped protect Texans from invasion.