Hadl was the first KU player to be picked twice for All-America (1960 and 1961) honors for his skills as a quarterback and halfback. Hadl also excelled as a defensive back, and punt returner and punter; he led the country with a 45.6-yard punting average in 1959. Hadl's No. 21 jersey is one of only three KU has retired. Hadl, who was picked for the all-conference team for three seasons, wound up with 1,281 yards passing and 1,016 yard rushing. Hadl still holds two KU records: Longest interception return, a 98-yard run against TCU; and longest punt, 94 yards vs. Oklahoma.
With Hadl running the offense, the Jayhawks were ranked in the top 20 during his junior and senior years, finishing 15-5-2. He wrapped up his KU career leading his team to a 33-7 win over Rice University in the Bluebonnet Bowl. He was also named MVP in the East-West Shrine game and the College All-Star Game.
He played at 6 ft 1 in (1.85 m) and 210 lb (95 kg). He 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 touchdowns per (14-game) season for the next four years.
He was the American Football League's leading passer in both 1965 and 1968, and was a four-time AFL All-Star. In 1969, he was selected as the AFL All-Star Game's Most Valuable Player. The other half of the Chargers' potent passing/receiving tandem was Lance Alworth, the first American Football League player to be inducted into the Pro Football Hall of Fame. Many observers believe that Hadl, who threw most of the passes that Alworth received, also belongs in the Hall.
Before the 1973 season, Hadl was traded to the Los Angeles Rams for defensive endCoy Bacon and running back Bob Thomas. Leading the Rams to the playoffs that year, he was named the National Football Conference Player of the Year. Hadl was traded to the Green Bay Packers the next season when he was beaten out for the starting quarterback position by James Harris.
Hadl finished with a starting record of 82-76-9 in his professional career. He holds the NFL record for the most tied games (9) by a starting quarterback.
Hadl wore #21 for all of his NFL career, and was the last regular starting quarterback to wear a uniform number greater than #19 before the NFL adopted a rigid uniform numbering system in 1973.
Hadl was also head coach of the Los Angeles Express football team of the USFL during 1984 and 1985, compiling a record of 13-23 in the regular season, 1-1 in the postseason.
Hadl is currently an associate athletic director in the University of Kansas athletic department, despite the belief that he might have been the "unnamed assistant football coach" at KU who provided improper benefits to recruits in the early 1980s in violation of NCAA regulations. Hadl has consistently denied any wrongdoing, and his current tenure with Kansas athletics has not yielded any suggestion of rules violations.