Henry McCarty was born in New York City to Irish Catholic parents, Catherine and Patrick McCarty. According to one theory, he was born on September 17, 1859, in New York City. On September 28, 1859, he was named "Patrick Henry McCarthy" at the Church of St. Peter. While his birth year has been confirmed as 1859, his exact birth date has been challenged as either September 17 or November 23 of that year. According to a letter from an administrator at Saint Peter's Church in Manhattan, the church has records proving McCarty was christened there on September 28, 1859. According to census data, his younger brother, Joseph McCarty, was born in 1863.

Catherine McCarty and her sons relocated to Indianapolis, Indiana, when her husband Patrick died, where she met William Henry Harrison Antrim. In 1870, the McCartys relocated to Wichita, Kansas, with Antrim. Catherine married Antrim on March 1, 1873, in the First Presbyterian Church in Santa Fe, New Mexico Territory, after moving a few years earlier; McCarty and his brother Joseph were witnesses to the event. Soon after, the family relocated from Santa Fe to Silver City, New Mexico, where Joseph McCarty adopted the surname Antrim. William Antrim abandoned the McCarty boys, leaving them orphans, shortly before McCarty's mother, Catherine, died of tuberculosis on September 16, 1874. Therefore, after his mother died, he became an orphan.

One of the interesting facts about Billy the Kid is that his first crime is stealing clothing and guns. When McCarty's mother died, he was 15 years old. In exchange for labor, Sarah Brown, the owner of a boarding house, provided him with accommodation and board. McCarty was arrested for stealing food on September 16, 1875. McCarty and George Schaefer stole a Chinese laundry ten days later, seizing garments and two firearms. McCarty was arrested and charged with theft. He fled two days later and became a fugitive, according to the Silver City Herald, which published the first story about him the next day. McCarty tracked down his stepfather and remained with him until Antrim evicted him; McCarty stole clothing and guns from him. It was the last time the two of them saw each other.

McCarty moved to southeastern Arizona Territory after leaving Antrim, where he worked as a ranch hand and spent his wages in adjacent gaming halls. Henry Hooker, a well-known rancher, recruited him as a ranch hand in 1876. During this period, McCarty met John R. Mackie, a Scottish-born criminal and former US Cavalry private who lingered near the US Army garrison at Camp Grant in Arizona after his discharge. Soon after, the two men began stealing horses from the local military. Because of his youth, tiny build, clean-shaven appearance, and disposition, McCarty earned the moniker "Kid Antrim".

It is a fact that he was famous for the Battle of Lincoln. Billy the Kid ultimately established his reputation as a gunslinger in 1878. A frontier conflict broke out at this time between a rancher called John Tunstall and two Irish tycoons named James Dolan and Lawrence Murphy. Tunstall learned of the posse's presence on his estate on February 18, 1878, and rode out to interfere. During the fight, one of the posse members shot Tunstall in the chest, knocking him from his horse. Another posse member snatched Tunstall's gun and shot him in the back of the head. Tunstall's assassination sparked the Lincoln County War, a fight between two factions.

On the night of July 14, McSween and the Regulators, now a party of fifty or sixty men, traveled to Lincoln and set up camp amid numerous structures. McCarty, Florencio Chavez, Jose Chavez y Chavez, Jim French, Harvey Morris, Tom O'Folliard, and Yginio Salazar were among those that visited the McSween home. Another group, led by Marin Chavez and Doc Scurlock, took up position on the top of a saloon. A nearby adobe bunkhouse was defended by Henry Newton Brown, Dick Smith, and George Coe.

On Tuesday, July 16, newly appointed sheriff George Peppin dispatched sharpshooters to the tavern to kill the McSween defenders. When one of the snipers, Charles Crawford, was killed by Fernando Herrera, Peppin's troops retreated. Peppin then requested help from Colonel Nathan Dudley, commandant of nearby Fort Stanton. Dudley refused to interfere in response to Peppin but eventually arrived in Lincoln with troops, tipping the battle in favor of the Murphy-Dolan group.

On Friday, July 19, a gun conflict erupted. McSween's followers gathered inside his home, and when Buck Powell and Deputy Sheriff Jack Long set fire to it, the residents began firing. McCarty and the other men fled the building as all but one of the apartments caught fire. Alexander McSween was shot and killed during the chaos by Robert W. Beckwith, who was subsequently shot and killed by McCarty.

One of the interesting facts about Billy the Kid is that he died when he was young. Governor Wallace set a new $500 bounty on McCarty's head while he was on the run. Responding to allegations that McCarty was in the neighborhood of Fort Sumner, Garrett departed Lincoln with two deputies on July 14, 1881, to investigate resident Pete Maxwell, a friend of McCarty's. Maxwell, the son of land baron Lucien Maxwell, spoke with Garrett for several hours the same day. Around midnight, the two were sitting in Maxwell's darkened bedroom when McCarty walked in unexpectedly.

The sequence of events is described differently by different people. According to the canonical version, McCarty failed to identify Garrett as he entered the room owing to bad lighting. Garrett pulled his revolver and fired twice after recognizing McCarty's voice. The first bullet struck McCarty just above his heart in the chest, while the second missed. Garrett's story is uncertain as to whether McCarty was killed quickly or over time.

A coroner's jury of six individuals was constituted by a local judge of the peace a few hours after the shooting. Members of the jury interrogated Maxwell and Garrett, and McCarty's body and the scene of the shooting were investigated. The jury certified the body as McCarty's, and the jury foreman told a local newspaper, "It was the Kid's body that we examined." McCarty was given a candlelight wake before being buried the next day, and his grave was marked with a wooden marker. Garrett traveled to Santa Fe, New Mexico, five days after McCarty's murder, to collect the $500 prize set by Governor Lew Wallace for his apprehension, dead or alive. The acting governor of New Mexico, William G. Ritch, declined to pay the reward. Residents of Las Vegas, Mesilla, Santa Fe, White Oaks, and other New Mexico cities raised almost $7,000 in prize money for Garrett over the next few weeks. The New Mexico territorial legislature enacted a special act a year and four days after McCarty's death to grant Garrett the $500 bounty reward promised by Governor Wallace.

One of the interesting facts about Billy the Kid is that his death has become a myth. Over time, legends arose suggesting that McCarty was not murdered and that Garrett fabricated the event and death out of the kindness in order for McCarty to avoid prosecution. Several men claimed to be Billy the Kid throughout the next 50 years. In 1948, a man from central Texas named Ollie P. Roberts, better known as Brushy Bill Roberts, claimed to be Billy the Kid and petitioned New Mexico Governor Thomas J. Mabry for a pardon. Mabry rejected Roberts' assertions, and Roberts died soon after. Nonetheless, Roberts' hometown of Hico, Texas, profited from his claim by building a Billy the Kid museum.

John Miller, a man from Arizona, also claimed to be McCarty. His family did not support this until 1938, sometime after his death. Miller's body was interred in the state-owned Arizona Pioneers' Home Cemetery in Prescott, Arizona; without permission from the state, Miller's teeth and bones were exhumed and analyzed in May 2005. DNA samples from the remains were transferred to a Dallas laboratory and examined to compare Miller's DNA with blood samples recovered from the old Lincoln County courthouse floorboards and a bench where McCarty's body was supposedly deposited after he was shot. According to a July 2015 Washington Post report, the lab results were useless.

In 2004, researchers attempted to exhume the remains of McCarty's mother, Catherine Antrim, whose DNA would be analyzed and compared to that of the body buried in William Bonney's grave. Her body had not been unearthed as of 2012.

Gale Cooper, an author, and amateur historian sued the Lincoln County Sheriff's Office under the Illinois Inspection of Public Documents Act in 2007 to get records of the results of the 2006 DNA tests and other forensic evidence acquired in the Billy the Kid investigations. In April 2012, 133 pages of documents were released; they gave no definitive proof verifying or refuting the widely believed tale of Garrett's murder of McCarty, but they did prove the existence of the records and that they might have been supplied earlier. Cooper received $100,000 in punitive damages in 2014, but the decision was later overturned by the New Mexico Court of Appeals. The lawsuit ended up costing Lincoln County approximately $300,000.