Robert LeRoy Parker was born on April 13, 1866, in Beaver, Utah Territory, the first of 13 children of English immigrants Maximillian Parker and Ann Campbell Gillies.

Butch Cassidy's Wild Bunch was one of the loosely organized outlaw gangs operating out of the Hole-in-the-Wall, a natural fortress in Wyoming with caves and a narrow entrance that was constantly guarded. The Wild Bunch claimed to make every attempt to abstain from killing people, and Cassidy boasted of having never killed a single man or woman in his entire career, but these claims were false. Kid Curry, "Flat-Nose" Curry, Will "News" Carver, and other members of the gang killed numerous people during their flight from law enforcement. Kid Curry alone killed nine lawmen while with the gang, and another two civilians during shootouts.

His parents were Mormons who moved from England to America in the 1850s in response to Brigham Young's call for overseas members of the Church of Latter-day Saints to help establish communities in Utah.

During a brief apprenticeship with a butcher in Rock Springs, Wyoming, he earned the nickname "Butch", which stuck when he struck out to work on area ranches. He added the last name Cassidy in honor of a cattle thief named Mike Cassidy, who had befriended him.

Butch Cassidy's first criminal offense was minor. Around 1880, he journeyed to a clothier's shop in another town, but found it closed. He broke into the shop and stole a pair of jeans and some pie, leaving an IOU promising to pay on his next visit. The clothier pressed charges, but Cassidy was acquitted by a jury.

The Wild Bunch typically separated following a robbery and fled in different directions to confuse their pursuers, later reuniting at a predetermined location such as the Hole-in-the-Wall, Robbers Roost, or Fannie Porter's brothel in San Antonio.

On June 24, 1889, Cassidy pulled off his first bank robbery, when he and several companions made off with $21,000 (equivalent to $605,000 in 2020) from the San Miguel Valley Bank in Telluride, after which they fled to the Robbers Roost, a remote hideout in southeastern Utah.

In 1894, Cassidy was found guilty of stealing a horse worth $5 in Wyoming and sentenced to two years in the state penitentiary. He was convinced he'd been set up by ranchers who didn't want him around. (He may have been running a protection racket.) After 18 months behind bars he was released for good behavior, although reportedly not before promising the governor he'd leave Wyoming's ranchers alone.

Sisters Ann and Josie Bassett owned a ranch near Browns Park and often supplied the gang with fresh horses and beef. Both Bassett girls were romantically involved with members of the gang, including Cassidy, and both sometimes accompanied the gang to one of their hideouts, known as "Robbers Roost". Associations with ranchers like the Bassetts allowed the gang considerable mobility, giving them an easy resupply of fresh horses and supplies, and a place to hole up for a night or two.

In the late 1890s, posing as "Charles L. Carter," an alleged gunman on the run for murder, Charlie Siringo infiltrated Butch Cassidy's gang. Several members of the Wild Bunch were captured due to Siringo's efforts, and the two remaining leaders, Butch Cassidy and the Sundance Kid, were forced to flee to South America.