After playing halfback on both offense and defense at the University of Kansas as a sophomore, Hadl played quarterback for his last two years at Kansas, and was selected as the school's Player of the Century. He was an All-American at running back in 1960 and at quarterback in 1961.
The 6-foot-1, 210-pounder joined the American Football League's San Diego Chargers in 1962. He shared quarterbacking duties until 1966, when he became San Diego's starting quarterback, and averaged over 3,000 yards and 23 tds per (14-game) season for the next four years. He was the American Football League's leading passer in 1965 and 1968, and the 1969 AFL All-Star Game Most Valuable Player. He is ranked as the third leading passer in league history, behind Hall of Famers Jack Kemp and George Blanda.