In 1968, McLain became the last Major League Baseball pitcher to win 30 or more games during a season (with a record of 31–6) — a feat accomplished by only 11 players in the 20th century, with the most recent 34 years earlier.

While achieving success in baseball, McLain faced legal issues in his personal life, including convictions related to organized crime and embezzlement, resulting in prison time.

[4] Upon his graduation from high school in June 1962, McLain was signed by the Chicago White Sox as an amateur free agent, and was assigned to play with the Harlan Smokies of the Appalachian League.

[6] McLain's minor league debut was notable for throwing a no-hitter and striking out 16 batters in a game against the Salem Rebels on June 28.

McLain is one of only six teenage pitchers to hit a major-league home run since 1920, a list that includes Hall-of-Famers Don Drysdale and Jim Palmer.

[4] In 1966, McLain had a 13–4 mid-season record and earned the role of American League starting pitcher in the 1966 All-Star Game, where he threw just 28 pitches to retire all nine batters that he faced.

[4] Going into the final game of the season against the California Angels, the Tigers needed a victory to force a one-game playoff with the Red Sox for the American League pennant.

[20] After the Tigers had clinched the 1968 American League pennant, McLain attracted attention during his 31st and final regular season victory in a game against the Yankees on September 19.

[4] McLain had grown up idolizing New York Yankee center fielder Mickey Mantle, who entered the game tied with Jimmie Foxx for third place in the major-league career home runs list.

[21] When Mantle—who was nine days away from his last major league appearance—came to bat in the eighth inning with the Tigers leading 6–1, McLain intentionally threw a soft pitch directly over home plate.

[23] After the game, McLain smilingly denied that he had served up an easy pitch for Mantle to hit; however, he was later reprimanded by Major League Baseball Commissioner William Eckert.

[4][21] McLain completed a 31–6 record along with a 1.96 earned run average, as the Tigers won the American League pennant by 12 games.

[12] After the record home-run year by Roger Maris in 1961, the major leagues increased the size of the strike zone from the top of the batter's shoulders to the bottom of the knees.

In the National League, Gibson posted a 1.12 earned run average, the lowest in 54 years, while Los Angeles Dodgers pitcher Don Drysdale threw a record 58+2⁄3 consecutive scoreless innings during the 1968 season.

[36] McLain had disagreements with Tigers' manager Mayo Smith, including disputes over the firing of pitching coach Johnny Sain.

[4] Meanwhile, McLain and his teammate Jim Northrup schemed together to make more money; they were back in Detroit furthering a plan that they shared to generate a nude baseball model calendar.

On October 9, 1970, the Tigers traded McLain, Elliott Maddox, Norm McRae, and Don Wert to the Washington Senators for Joe Coleman, Eddie Brinkman, Jim Hannan, and Aurelio Rodríguez.

Kuhn later wrote in his autobiography, Hardball: The Education of a Baseball Commissioner, that he was shocked at what he called a "foolish gamble" by the Senators, and predicted that the trade would turn out to be a Tiger heist.

[40] The feeling was mutual; early in the 1971 season, McLain became a charter member of the "Underminers' Club", a group of five players dedicated to getting Williams fired.

After the 1971 season, McLain was traded to the Oakland Athletics for journeyman pitcher Jim Panther and prospect Don Stanhouse (who went on to have a few good years as the Baltimore Orioles' closer in the late 1970s).

His final major league appearance came on September 12 against the Cincinnati Reds; he came into a tied game in the ninth and promptly gave up three runs without retiring a batter, taking the loss.

(Coincidentally, the last batter McLain ever faced in the major leagues was Pete Rose, who also was involved in a gambling scandal years later.

[4] McLain's success on the playing field led to endorsement opportunities from the Hammond Organ Company; he also made musical appearances in Las Vegas.

[45] He was invited to appear with his musical quartet on The Ed Sullivan Show along with his World Series opponent (and guitarist) Bob Gibson.

[50] McLain also earned quite a bit of money hustling golf, easily attracting "marks" due to his past baseball fame.

[citation needed] In 1996, he was convicted on charges of embezzlement, mail fraud, and conspiracy in connection with the theft of $2.5 million from the Peet employees' pension fund.

In 2007, McLain released his autobiography I Told You I Wasn't Perfect, co-authored by longtime Detroit sportscaster and author Eli Zaret.

[55] On April 11, 2008, McLain was arrested without incident after deputies discovered an outstanding warrant against him for failing to appear for a January 16 court hearing.

[56] On September 22, 2011, McLain was arrested in Port Huron, Michigan at the Canada–United States border after officials discovered an outstanding warrant against him from St. Charles Parish, Louisiana.

[58] In January 2019 McLain and former local sportscasters Eli Zaret and Bob Page launched a podcast called No Filter Sports.