Born Robert Clay Allison, he was the fourth of the nine children of Jeremiah Scotland Allison and his wife Mariah R. Brown. His father, a Presbyterian minister, also raised cattle and sheep to support his family. Clay Allison reportedly was restless from birth; as a young man, he became known for wild mood swings and his quick temper.
American Civil War
Allison helped on the family farm near Waynesboro, Tennessee, until the American Civil War began when he was 21. On October 15, 1861, he enlisted the Confederate States Army in Captain W. H. Jackson's Artillery Battery. Three months later, however, he was medically discharged because an old head injury: "Incapable of performing the duties of a soldier because of a blow received many years ago. Emotional or physical excitement produces paroxymals of a mixed character, partly epileptic and partly maniacal."
On September 22, 1862, Allison enlisted in the 9th Tennessee Cavalry Regiment, where he served under the Confederate "wizard of the saddle", General Nathan Bedford Forrest. At the war's end, he surrendered with Forrest's men on May 4, 1865, at Gainesville, Alabama. After briefly being held prisoner of war, Allison and the others were paroled on May 10 and allowed to return home.
After the war
Once back home, Allison was involved in several violent confrontations before he left for Texas. A popular - but probably untrue - story claims that when a corporal from the 3rd Illinois Cavalry Regiment arrived at the Allison family's farm with intention of seizing it, after a rude confrontation and the breaking of his mother's vase (an anniversary present from his father) Clay Allison took a rifle from the house and killed him. Whatever the reason, Clay Allison, along with his brothers Monroe and John, and sister Mary and her husband Lewis Coleman, soon moved to Texas to settle.
In the fall of 1870, a man named Charles Kennedy was being held in the local jail in Elizabethtown, accused of going mad and murdering several strangers and his own daughter. A mob led by Clay Allison broke into the jail, took Kennedy from his cell, and hanged him. When the house of Kennedy was later searched they found the bodies of those he had killed along with his daughter. Allegedly, Allison then cut off the man's head and carried it in a sack 29 miles (47 km) to Cimarron, where he placed it on display on a pole in front of the St. James Inn. (Though Charles Kennedy died at the hands of Clay; his head could not have been taken to Lambert's saloon as the saloon did not yet exist at that date.) 
Allison was involved in numerous encounters during this period, often in man-to-man knife fights. He believed himself fast with a gun, but this changed when he was outdrawn in a friendly competition with Mason Bowman. Bowman and Allison became friends, and Mace Bowman is said to have helped Allison to improve his fast draw skills.
Notoriety as a gunfighter
On January 7, 1874, Allison killed a gunman named Chunk Colbert. After they had raced their horses, they entered the Clifton House, an inn located in Colfax County, New Mexico, where they sat down together for dinner. Colbert had already killed seven men and had quarreled with Allison years earlier. (Allison had beaten Colbert's uncle, Zachary Colbert, when he tried to overcharge Allison for the ferry across the Brazos River.) During their meal, Colbert suddenly tried to draw his pistol to shoot Allison; however, the barrel struck the table. Allison then drew his own revolver and fired one shot, striking Colbert in the head. Asked why he had accepted a dinner invitation from a man likely to try to kill him, Allison replied, "Because I didn't want to send a man to hell on an empty stomach". Allison's reputation as a gunman grew, as did his notoriety.
On October 30, 1875, Allison is alleged to have led a lynch-mob to kill Cruz Vega, who was suspected of murdering the Reverend F.J. Tolby, a Methodist circuit-rider. The mob hanged the man from a telegraph pole near Cimarron. On November 1, Vega's family members, led by Vega's uncle Francisco Griego, began making threats of revenge. They went to the Lambert Inn (now the St. James Hotel), where they confronted Allison and accused him of taking part in the lynching. Griego reached for his revolver. Allison was faster and shot Griego twice, killing him. On November 10, Allison was charged with the murder of Francisco Griego, but after an inquiry the charge was dropped and the shooting was ruled self-defense.
In December 1876, Allison and his brother John rode into Las Animas, Colorado, where they stopped at a local saloon. Constable Charles Faber of Bent County told the Allisons they should surrender their pistols, as an ordinance made it illegal to carry weapons inside the town limits. When the Allisons refused, Sheriff Faber left. He deputized two men and returned with them to the saloon. When the posse stepped inside, someone yelled, "Look out!" The sheriff and his men promptly opened fire. John Allison was hit three times - in the chest, arm, and leg. Clay Allison turned and fired four shots, killing Sheriff Faber. The deputized men fled. Allison chased after them, but they escaped. Both Allison brothers would be arrested and charged with manslaughter, but the charge was dismissed as the sheriff had begun the fight. This gunfight more than any raised Clay Allison to legendary status.
Alleged confrontation with Wyatt Earp
In March 1877, Allison sold his ranch to his brother, John. He relocated to Sedalia, Missouri, the birthplace of his wife and sister-in-law. Clay next moved to Hays City, Kansas, where he established himself as a cattle broker. By the time Allison arrived in Dodge City, Kansas, his reputation had preceded him. Nevertheless, several cowboys working for him apparently were mistreated by the local marshal's office. Dodge City was a "cattle town", and laws were upheld by force. The deputy marshal at the time was a man later to gain fame himself: Wyatt Earp.
Earp's biographer and Earp himself claimed that Wyatt Earp and his friend Bat Masterson confronted Allison and his men in a saloon, and that Allison backed down before them. However Masterson was not in town at the time and there is no evidence the encounter ever took place.
According to contemporary accounts, a cattleman named Dick McNulty and Chalk Beeson, owner of the Long Branch Saloon, convinced Allison and his cowboys to surrender their guns. Wyatt Earp did not make his claim until after Allison's death. Charlie Siringo, a cowboy at the time but later a well known Pinkerton Detective, had witnessed the incident and left a written account. Siringo agreed that it was McNulty and Beeson who ended the incident, and said Earp had not even approached Clay Allison.
Allison ranched from 1880 to 1883 with his brothers, John William Allison and Jeremiah Monroe Allison. Their ranch was 12 miles northeast of Mobeetie, at the junction of the Washita River and Gageby Creek, in what was then Wheeler County, Texas (now Hemphill County, Texas ). A verified story tells how a totally drunk Clay Allison rode through Mobeetie stark naked one day, wearing only his holster and revolver. On February 15, 1881, Allison married America Medora McCulloch in Mobeetie and became a family man.
By 1883, Allison had sold his ranch and moved to Pope's Wells, purchasing another ranch near the Pecos River crossing of the Texas-New Mexico line (this was a landmark on the Goodnight-Loving Trail), 50 miles northwest of Pecos, Texas.
Clay and his wife "Dora" had two children: Patti Dora Allison (married name Byars), was born on August 9, 1885, in Peña Flora district of Colfax County; she died on August 21, 1971, in Fort Worth, Texas. Clay Pearl Allison (married name Parker), was born on February 10, 1888 (seven months after her father’s death), Pecos, Texas; she died on November 21, 1962.
Clay Allison died an accidental death while working on his ranch. On July 3, 1887, Allison was hauling a wagon load of supplies when the load shifted. A sack of grain fell from the wagon, and Allison fell from the wagon as he tried to catch it. A wagon wheel rolled over him, breaking his neck. His death was almost instantaneous; he was 46 years old.
Robert Clay Allison was buried the next day in Pecos Cemetery, in Pecos, Texas. It is said that hundreds attended his funeral, either to pay their respects or simply out of curiosity.
Dora McCullough Allison married to Jesse Lee Johnson, in Pecos, Texas, on October 23, 1890. She moved with him to Fort Worth in 1897. Allison's widow died on January 18, 1926 in Baltimore, Maryland and was interred in Oakwood Cemetery in Fort Worth.
In a special ceremony held on August 28, 1975, Clay Allison's remains were re-interred at Pecos Park, just west of the Pecos Museum.
His grave marker reads:
ROBERT C ALLISON
9th TENN CAV
SEP 2 1840
JUL 3 1887
A second marker was later placed at the foot of the grave: "He never killed a man that did not need killing".
In popular culture
Before being cast as Bart Maverick on the ABC Western television series Maverick, Jack Kelly appeared as Clay Allison in the 1955 syndicated series Stories of the Century, starring and narrated by Jim Davis.
On September 25, 1956, Myron Healey played Clay Allison in an episode of the same name of the ABC/Desilu western series, The Life and Legend of Wyatt Earp, with Hugh O'Brian in the title role. In the story line, Pete Albright, a storeowner in Dodge City, Kansas, played by Charles Fredricks tries to hire Allison to gun down Earp because the marshal is fighting crime in the town and costing merchants business in the process. Allison will not take money but is willing to challenge Earp until he is overcome by his own drunkenness. In another episode of the same series, the actor Mike Ragan played Allison, who joins other Wild West figures, including Ben Thompson (Denver Pyle) and John Wesley Hardin (Phillip Pine), in coming to Earp's aid in a shootout with the owner and foreman of the Big T Ranch, Rance Purcell (Richard Devon) and Gus Andrews (Grant Withers).
- Cunningham, Sharon. – "The Allison Clan – A Visit"'. – Western Outlaw. – (Adobe Acrobat *.PDF document).
- US Census, 1860, Wayne Co., TN, page 197, family #1347
- [this report may be urban Legend; according to  the 3rd Illinois Cavalry had 1 officer KIA in 1863 at Vicksburg and 7 others officers died from non combat causes!Likewise although the Regiment was in Tennessee in 1864-7 enlisted men were killed in Skirmishes; the regiment was not in Tennessee postwar as it was mustered out in October 1865!]
- Clay Allison – Page 2 – Legends of America
- Allison from the Handbook of Texas Online
- Hotel St. James history
- Clay Allison – Page 4 – Legends of America
- See Colbert's entry – Legends of America
- Revenge Tech Wild West Tech
- In June 1875 Griego had killed a soldier and wounded two others
- Constable Charles Faber – the Officer Down Memorial Page
- Clay Allison – Page 1 – Legends of America
- U.S. Geological Survey Geographic Names Information System: Gageby Creek, Texas
- Clay Allison – Page 5 – Legends of America
- John Pope from the Handbook of Texas Online
- "America Medora "Dora" McCulloch Johnson". Find a Grave. Retrieved October 20, 2014.
- The Second Marker
- ""Clay Allison", September 25, 1956". Internet Movie Data Base. Retrieved March 26, 2014.
- ""The Time for All Good Men" (June 4, 1957)". Internet Movie Data Base. Retrieved April 18, 2014.
- "Clay Allison, Tales of Wells Fargo, June 15, 1959". Internet Movie Data Base. Retrieved February 18, 2013.
- True Story of Clay Allison and Wyatt Earp, Dodge City
- Clay Allison from the Handbook of Texas Online
- Clay Allison at Find a Grave
- Footstone which reads: "He never killed a man that did not need killing"