Robert LeRoy Parker was born in Beaver, Utah to Maximillian Parker and Ann Campbell Gillies, English Mormon and Scottish immigrants to the Utah Territory. His parents were residents of Victoria Road, Preston, Lancashire but fled England due to religious persecution for their Mormon faith. He grew up on his parents' ranch near Circleville, Utah, some 215 miles south of Salt Lake City, Utah.

Parker left home during his early teens when, while working at a local dairy farm, he fell in with Mike Cassidy, a horse thief and cattle rustler. He subsequently worked several ranches in addition to a brief stint as a butcher in Rock Springs, Wyoming, whence he acquired for himself the nickname "Butch", (however a "Butch" is also the name given to a borrowed gun) to which he soon appended the surname Cassidy in honor of his old friend and mentor.