Thornbury was born in 1948[3] and started lassoing at the age three,[4] and stated there were "No computer games, no TV, no electronics — just the rope I grew up on." His father, J.D., was a trick horse rider who raised him traveling from rodeo to rodeo as his family performed on a Midwest circuit.[5]
As an adult, Thornbury first learned saddlery in Michigan but fine-tuned his art and tooling skills later from a Pimasaddler named Mervyn Ringlero. Thornbury moved to California in the 1970s and became popular worldwide for his saddles and leather goods that included work for stuntmen in Hollywood.[5] Thornbury continued in the rodeo as a Bronc rider, and at one point, was hired to model for Marlboro Man ads.[6]