He demonstrated exemplary patience as a batter, producing a career .394 on-base percentage (OBP) and twice leading the major leagues in bases on balls.

A right-handed batter and thrower, McGwire stood 6 feet 5 inches (1.96 m) tall and weighed 245 pounds (111 kg) during his playing career.

With the Cardinals in 1998, McGwire joined Cubs slugger Sammy Sosa in a chase for the single-season home-run record set by Roger Maris in 1961.

He attended Damien High School in La Verne, California, where he played baseball, golf, and basketball.

[5] McGwire played college baseball at the University of Southern California, where he was a teammate of Randy Johnson, Jack Del Rio, and Rodney Peete under coach Rod Dedeaux.

[9] Three days later, McGwire broke the major-league record of 38, which Frank Robinson and Wally Berger had jointly held.

In September, McGwire hit nine more home runs while posting monthly personal bests of a .351 batting average, .419 on-base percentage (OBP) and 11 doubles (2B).

McGwire's 49 home runs as a rookie stood as a major league record until Aaron Judge hit 52 for the New York Yankees in 2017.

[12][13] Through May 2009, McGwire was tied for third all-time with Joe DiMaggio in home runs over his first two calendar years in the major leagues (71), behind Chuck Klein (83) and Ryan Braun (79).

[14] McGwire's most famous home run with the A's was likely his game-winning solo shot in the bottom of the ninth inning of Game 3 of the 1988 World Series against the Los Angeles Dodgers and former A's closer Jay Howell.

Manager Tony La Russa sat him for the final game of the season to avoid causing his batting average to dip below .200.

Despite the declining averages during this time of his career, McGwire's high base-on-balls totals allowed him to maintain an acceptable on-base percentage.

McGwire stated in an interview with Sports Illustrated that 1991 was the "worst year" of his life, with his on-field performance and marriage difficulties, and that he "didn't lift a weight" that entire season.

[17][18] The "new look" McGwire hit 42 homers and batted .268 in 1992, with an outstanding OPS+ of 175 (the highest of his career to that point), and put on a victorious home-run-hitting show at the Home Run Derby during the 1992 All-Star break.

On April 21, 1997, McGwire became the fourth and final player to hit a home run over the left-field roof of Detroit's Tiger Stadium, joining Harmon Killebrew, Frank Howard and Cecil Fielder.

[20] On July 31, having already amassed 34 home runs in the 1997 season, McGwire was traded from the Oakland Athletics to the St. Louis Cardinals for T. J. Mathews, Eric Ludwick and Blake Stein.

He also finished third in the major leagues in slugging percentage (.646), fourth in OPS (1.039), fifth in OPS+ (170), tenth in RBI (123), and ninth in walks (101).

It is also believed that McGwire later encouraged Jim Edmonds, another Southern California resident who was traded to St. Louis, to forgo free agency and sign a contract with the Cardinals in 2000.

[22] On September 8, 1998, McGwire hit a pitch by the Cubs' Steve Trachsel over the left-field wall for his record-breaking 62nd home run, setting off massive celebrations at Busch Stadium.

That squad, which included future Hall-of-Famer Barry Larkin, emerged as the favorite for the competition, after Cuba joined the Soviet Union-led boycott of the games.

[35] In early November 2012, McGwire rejected a contract extension to return as Cardinals hitting coach for the 2013 season.

[42] Considered one of the slowest runners in the game, McGwire had the fewest career triples (six) of any player with 5,000 or more at-bats, and had just 12 stolen bases while being caught stealing eight times.

A five-mile stretch of Interstate 70 in Missouri in St. Louis and near Busch Stadium was named Mark McGwire Highway to honor his 70-home-run achievement, along with his various good works for the city.

In May 2010, St. Louis politicians succeeded in passing a state bill to change the name to Mark Twain Highway.

In a 1998 article by Associated Press writer Steve Wilstein, McGwire admitted to taking androstenedione,[45] an over-the-counter muscle enhancement product that had already been banned by the NFL[46][47], and the IOC;[48] however, use of the substance was not prohibited by Major League Baseball at the time, and it was not federally classified as an anabolic steroid in the United States until 2004.

[49] Jose Canseco released a book, Juiced: Wild Times, Rampant 'Roids, Smash Hits & How Baseball Got Big, in 2005.

During his testimony on March 17, 2005, McGwire declined to answer questions under oath when he appeared before the House Government Reform Committee.

In a tearful opening statement, McGwire said: Asking me or any other player to answer questions about who took steroids in front of television cameras will not solve the problem.

Prior to admitting to using steroids, McGwire avoided the media and spent much of his free time playing golf.

[57] He also worked as a hitting coach for Major League players Matt Holliday, Bobby Crosby, Chris Duncan and Skip Schumaker.