Selected 10th overall in the first round of the 1962 AFL Draft out of Kansas, where he was a two-time All-American, John Hadl chose Sid Gillman’s San Diego Chargers over the NFL’s Detroit Lions. In the Chargers’ explosive offense, Hadl initially split time and backed up veteran Tobin Rote during his first two seasons, helping San Diego win the 1963 AFL Championship with a dominant 51–10 victory over Boston.

After becoming the primary starting quarterback in 1964, Hadl quickly justified Gillman's faith in him. That season, he earned his first AFL All-Star selection and led the Chargers back to the AFL title game. In 1965, he led the AFL in passing yards (2,798) and earned his second All-Star nod. Hadl continued to build on that production, throwing for 2,846 yards in 1966 and 3,365 in 1967. In 1968, he led the league in both passing yards (3,473) and touchdown passes (27), while also topping the interception leaderboard with 31 in his high-risk, high-reward passing style.

Hadl’s deep-ball prowess made the Chargers' offense one of the most feared aerial attacks in pro football history. Pairing with Hall of Fame receiver Lance Alworth, Hadl mastered the deep post and wheel routes that defined Air Coryell’s predecessor. In 1971, Hadl delivered another stellar campaign, once again leading the league in passing yards (3,075) and touchdown passes (21). He closed out his 11-year run in San Diego with a Pro Bowl campaign in 1972, demonstrating his staying power through the AFL-NFL merger.

Traded to the Los Angeles Rams in 1973, Hadl earned First-Team All-Pro honors in LA before finishing his 16-year career with the Packers and Oilers. Yet his legacy remains forever anchored in Southern California, where his #21 jersey and high-flying arm helped build the Chargers' modern offensive identity.

In 1983, the franchise officially recognized his massive contributions by inducting Hadl into the Chargers Hall of Fame.  As a Charger, Hadl threw for 26,938 passing yards, 201 passing touchdowns, 211 interceptions, and 1,241 rushing yards with 16 rushing touchdowns.