Gastineau was born in Ardmore, Oklahoma, and moved to Springerville, Arizona at the age of seven, when his parents, Ernie and Lou, bought a ranch. Mark broke his leg as a child in such a horrific manner that he was told by doctors that he would never walk again. Mark overcame the injury and the odds and Ernie built his son a rodeo ring, and Mark began entering team-roping events at 12. Mark's other passion was collecting Indian artifacts in Arizona's White Mountains. At Round Valley High School, Gastineau needed urging from his father to play football. Gastineau showed promise, but not enough to attract attention from major colleges.