After the Mexican War and the annexation of Texas by the United States, the population of the state began to increase rapidly, but the increasing numbers were crowded into a limited area because Indians controlled the majority of the state. To open new areas and provide protection for settlers, in 1848 the United States War Department authorized a line of army forts from the Rio Grande to the Red River. Fort Mason's location on Post Oak Hill near Comanche and Centennial creeks in the northern part of what was then called Gillespie County was chosen by Lt. Col. William J. Hardee, assisted by Richard Austin Howard, on July 6, 1851. Hardee left the actual establishment of the post to Bvt. Maj. Hamilton W. Merrill and companies A and B of the Second Dragoons. The post was most likely named either for Lt. George T. Mason, who was killed at Brownsville during the Mexican War, or for Gen. Richard Barnes Mason, who died only a year before the fort was established.
During the next ten years, or until the beginning of the Civil War, the fort played an important part in settlement of the area. Settlers at first stayed close to the fort, but as the aggressive attitude of the military became apparent, additional settlers located farther from the post. The Indians-Kiowas, Lipan Apaches, and Comanches-were driven farther away and began making fewer raids into the settlements. The fort was closed several times during that decade, first in January 1854. It was reoccupied by Company A, First Dragoons, from March to May and was occupied by various companies of the Second United States Cavalry from January 14, 1856, to March 29, 1861, when it passed into the hands of the secessionists. The fort reached its maximum population in January 1856, when the headquarters and companies B, C, D, G, H, and I of the Second Cavalry were all stationed there, with Col. Albert Sidney Johnston in command. Twenty officers stationed at Fort Mason before the Civil War became generals. Twelve fought for the Confederacy, eight for the Union. Among these generals were Earl Van Dorn, Fitzhugh Lee, E. Kirby Smith, George H. Thomas, Robert E. Lee, John Bell Hood, William J. Hardee, and Philip St. George Cooke. Fort Mason was designated regimental headquarters for the Second Cavalry several times.
For a short period during 1862 the Confederate Army held 215 men prisoner, mostly civilians accused of being Union sympathizers, in the fort. During August 1862 they were marched to Austin. Indian depredations during the Civil War and immediately afterward were worse than they had ever been. The area was terrorized by killings, thefts, and nuisance raids. Texas state troops and minutemen had been unable to cope with the problem. The United States Army, in the form of the Headquarters Company with field staff and officers and the regimental band and Company F of the Fourth United States Cavalry, reoccupied the fort on December 24, 1866. Gen. John Porter Hatch was the commanding officer.
The fort was repaired and improved through the use of civilian artisans and military labor. Reconstruction lawlessness also affected military personnel; a large number of desertions and courts-martial were reported. Cavalry were replaced with soldiers from the Thirty-fifth Infantry over a period of time. The last inspection of the fort occurred on January 13, 1869. The report listed twenty-five buildings, mostly vacant and in need of repair; only sixty-nine men were present. The order to close the fort was carried out on March 23, 1869.
During 1870 the state of Texas organized several companies of frontier forces. Fort Mason was reopened in September of that year as headquarters for Companies A and B, Frontier Forces. Capt. James M. Hunter, later county judge of Mason County, was in command for most of that year. During the next year the forces were disbanded or moved, and for the last time the fort was closed.
Although the fort buildings and land became private property, the rock buildings were gradually dismantled by local citizens. Many early homes in the town of Mason contained material from the fort. In 1975 a local group of citizens began reconstructing one of the officers' quarters at the site. Many people contributed and assisted in the building. Today, this building belongs to the Mason County Historical Society.