The Eighteenth Texas Cavalry, also known as Darnell's Cavalry Regiment, was organized at Dallas, Texas, on March 15, 1862. The regiment initially had ten companies lettered A to K comprising men from the counties of Bastrop, Bell, Dallas, Denton, Ellis, Harris, Henderson, Johnson, Travis, and Williamson. Commanding the regiment was Col. Nicholas H. Darnell with field officers Lt. Col. John T. Coit, Maj. Charles C. Morgan, and Maj. William A. Ryan. The regiment was first assigned to duty in the Indian Territory where it became involved in a dispute with that area's commanding officer Gen. Albert Pike. The dispute stemmed from the regiment moving its camp without notifying General Pike. Soon after, the regiment transferred to Arkansas Post, Arkansas, and General Paul O. Hébert recommended dismounting the regiment in July 1862. The regiment fought in the battle of Arkansas Post that started on January 4, 1863, and was captured by Union forces at the conclusion of fighting on January 11. Company H was assigned to the prison camp in Little Rock at the time of the capture.
In April 1863 most of the men serving in the regiment were exchanged and returned to Confederate service. From May 1863 to war's end the Eighteenth Texas Cavalry served in the Army of Tennessee. By July 1863 the Eighteenth Texas Cavalry Regiment was consolidated with the Seventeenth Texas Cavalry Regiment. Additional consolidations throughout the war took place with the Twenty-fourth and Twenty-fifth Texas Cavalry regiments.
The Eighteenth Texas Cavalry took part in more than thirty engagements and battles while serving in the Army of Tennessee. Its most notable battles and campaigns were Chickamauga on September 19–20, 1863; the Chattanooga siege and Chattanooga from September to November 1863; the Atlanta campaign from May to September 1864; Jonesboro from August 31 to September 1, 1864; Franklin on November 30, 1864; Nashville on December 15–16, 1864; the Carolinas campaign from February to April, 1865; and Bentonville on March 19–21, 1865. The Eighteenth Texas Cavalry suffered heavy casualties throughout the war and probably fewer than 125 enlisted men and officers were present at the regiment's surrender on April 26, 1865, at Bennett's House, Durham Station, in North Carolina.