Hadl was the first Kansas 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, 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 Kansas has retired. Hadl, who was picked for the all-conference team for three seasons, wound up with 1,281 yards passing and 1,016 yards rushing. Hadl still holds two Kansas records: longest interception return, a 98-yard run against TCU; and longest punt, 94 yards versus 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 Kansas career leading his team to a 33–7 win over Rice in the Bluebonnet Bowl. He was also named MVP in the East–West Shrine Game and the College All-Star Game.
Hadl 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.
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—his first playoff appearance of any sort since 1965—he was named the National Football Conference Player of the Year. In the following season, after he was beaten out for the starting quarterback position by James Harris, he was traded to the Green Bay Packers for five draft picks—first and second round picks for 1975 and 1976, as well as a third round pick in 1975. Reportedly, head coach and general manager Dan Devine felt that an experienced quarterback was the only thing standing between the Packers and only their second playoff appearance since 1967. The trade turned out to be an unmitigated disaster; it is reckoned as one of the worst (if not the worst) trades for a starting quarterback in NFL history. Hadl played a total of 22 games with the Packers and threw for 9 touchdowns and 29 interceptions behind a porous offensive line; the team posted a 7-15 record over this span. The trade caused irreparable harm to Hadl's legacy and hastened a two-decade fall from glory for the Packer franchise; they would not make the playoffs again in a non-strike year until 1993. At the same time, the Rams used the picks acquired in the trade to acquire many of the players that allowed them to dominate the NFC West for the rest of the 1970s and lead them to an appearance in Super Bowl XIV. In later years, when asked for his thoughts on the infamous deal, Hadl himself expressed the surprise he felt, in 1974, at being sent to Green Bay: "I really didn't believe it... I didn't think anyone would be that desperate." Following the 1975 season, he was traded to another moribund franchise, the Houston Oilers, serving as backup to Dan Pastorini for two years before retiring.
Despite his tenure with Green Bay, 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 nearly his entire NFL career, aside from his first season with Green Bay when he briefly wore #12. He 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 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 Kansas 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.