Purdy was born on December 27, 1999, in Queen Creek, Arizona. He played high school football for Perry High School in Gilbert, Arizona. Purdy is the middle child of Shawn and Carrie Purdy. Shawn was a Minor League Baseball player for eight seasons. His older sister Whittney played softball at Southeastern University. His younger brother Chubba later became the starting quarterback at Perry before being recruited to Florida State as a dual-threat quarterback. As a youth, Purdy played quarterback in a flag football league. His father attributed Purdy's later success in part to the faster processing speed required to play flag football; quarterbacks were given seven seconds to dispose of the football before plays were blown dead.

Purdy started his varsity football career during the 2015 season as a sophomore for the newly opened Perry High School. In 2016, the Arizona Interscholastic Association (AIA) adjusted the athletic regions, placing Perry in the 6A Division Premier Region, ranked as the sixth toughest region in the nation. Purdy's performance elevated Perry into the 6A Division AIA State Championship game in 2016 and 2017, losing 65–28 and 48–42, respectively, both to Chandler High School.