Hunter was known as a skilled control pitcher and was one of the best big game pitchers during the early 1970's. He won 21 or more games five years in a row (1971-1975) and tied for the major league lead in wins in 1974 (25) and 1975 (23). Hunter also led the major leagues in Earned Run Average in 1974 while winning the American League Cy Young Award. Hunter earned five World Series championship rings, and threw a perfect game in 1968 (the first regular season perfect game in the American League in 46 years). Hunter was an 8-time All-Star.

He began his major league career playing for the Kansas City and Oakland Athletics, whose owner Charles O. Finley gave him the nickname "Catfish". By 1974 Hunter and Finley had fallen out. Finley refused to make statutory payments to Hunter's life insurance. The contractual wrangle was eventually referred to arbitrator Peter Seitz, who declared the contract breached, thus nullifying the standard reserve clause and making Hunter a free agent. As the first free agent of the modern era, he signed for New York Yankees for the then-unprecedented sum of $3.5 million, where his success continued, appearing in 3 further World Series, and winning 2. Hunter retired from baseball in 1979, at the age of 33. He was selected to the Baseball Hall of Fame in 1987. When asked to choose which team to go into the Hall of Fame as, Hunter notably refused to choose between the Athletics and Yankees, loving both franchises equally. His plaque is one of the few with no insignia on the cap. The Athletics retired his number 27 jersey.

Hunter died in Hertford, North Carolina, where he was born, after falling and hitting his head. He had been suffering from Lou Gehrig's Disease at the time. He was only 53 years old. In 1975, he was the subject of the Bob Dylan song, "Catfish."

Career statistics W L PCT ERA G GS CG SHO SV IP H ER R HR BB SO WP HBP 224 166 .574 3.26 500 476 181 42 0 3449 2958 1248 1380 374 954 2012 49 49