Just before the All-Star break in 1980 Brett was hitting .337 when he strained ankle ligaments trying to steal a base; however, he got hot in July and even hotter in August, going 4-for-4 on the 17th to reach the magic figure of .400. With two weeks left, he slumped but finished strong (10-for-19), ending the season at .390; he also led the league in slugging (.664) and, to absolutely no one's surprise, was named MVP.

That year, the Royals broke the playoff jinx against the Yankees as Brett homered in Game One and hit an upper deck home run off Goose Gossage to win Game Three. Although the Phils beat KC in a six-game Series, Brett homered and went 9-for-24 in the Series. The Royals finally won a championship in 1985 as Brett starred offensively and defensively; he went 4-for-5 in Game Seven, robbed the Cardinals of five potential hits and went 10-for-27, scoring five runs.

On July 24, 1983, Brett made history in a different way. With New York leading Kansas City 4-3 in the ninth inning at Yankee Stadium, he homered with two out and a man on to put the Royals in front, 5-4. Yankee manager Billy Martin contended that Brett should be declared out and the runs shouldn't count, because his bat had pine tar more than 18 inches from the knob to the barrel; the umpires agreed, awarding the win to the Yankees.

The Royals protested, however, and AL president Lee MacPhail ended the Great Pine Tar Incident by overruling his own umpires and declaring a resumption of play at a later date. Brett won his third batting title in 1990.

He retired after the 1993 season and became a member of the Royals' front office.