He played in Major League Baseball as a first baseman from 1998 to 2009, most prominently as a member of the Minnesota Twins where he was a Gold Glove Award winner.

After his playing career, Mientkiewicz briefly worked as a television sports analyst before serving as a minor league manager in the Los Angeles Dodgers and Detroit Tigers organizations.

Mientkiewicz attended Westminster Christian School in Palmetto Bay, Florida, where he was a teammate of Alex Rodriguez and Dan Perkins.

Mientkiewicz hit the go-ahead grand slam against South Korea in the semi-finals to help the U.S. capture its first-ever gold medal in baseball.

[10] His numbers dipped in 2002; however, he reached the post-season for the first time in his career, and hit two home runs in the 2002 American League Division Series against the Oakland Athletics.

[11] Chicago White Sox General Manager Kenny Williams shot back that Mientkiewicz should not worry about the game's location because he would not be there.

With infield defense proving to be their Achilles' heel, they made a four-team trade deadline deal on July 31 that landed Mientkiewicz and Montreal Expos shortstop Orlando Cabrera with the Boston Red Sox, and sent Justin Jones to the Twins.

The next day, July 31, 2004, Mientkiewicz was in the visiting dugout as a member of the Boston Red Sox, and started at first base and hit sixth in the lineup.

Mientkiewicz and Cabrera proved valuable additions to their new franchise as the Red Sox surged to within three games of the Yankees by the end of the season, and took the A.L.

On August 16, Mientkiewicz made an emergency start at second base, a position he had only ever played four times in the minor leagues, and never in the majors.

[15] In the final game of the 2004 World Series, Mientkiewicz was playing first when St. Louis Cardinals shortstop Édgar Rentería grounded back to pitcher Keith Foulke.

[16] As Boston had not won a World Series in 86 years, the ball symbolized the end of the so-called "Curse of the Bambino", and was of considerable interest to memorabilia collectors.

[28] On June 2, 2007, Mientkiewicz collided with Mike Lowell of the Boston Red Sox while trying to field a throw from shortstop Derek Jeter.

[34] On February 26, 2009, Mientkiewicz signed a minor league contract with an invitation to spring training with the Los Angeles Dodgers.

[35] He made the Major League roster as a pinch hitter and appeared in seven games for the Dodgers in April before dislocating his shoulder sliding into second base and being placed on the 60-day disabled list.

[36] After a brief rehab stint with the Triple-A Albuquerque Isotopes from July 28 to August 17, Mientkiewicz rejoined the Dodgers in September, seeing sporadic action as a pinch hitter down the stretch.

Mientkiewicz signed a minor league contract with the Dodgers for the 2010 season, and came to camp to compete for the left-handed pinch hitter role.

He was offered a coaching position with the team when he did not make the club out of spring training, but opted instead to keep playing and become a free agent.

[44] He was hired in 2020 as an on-air personality for ESPN's ACC Network alongside Gaby Sanchez, Javier Lopez, and Adam Greenberg.

After 2012, he was hired by the Minnesota Twins organization as the manager of the high-Class A Fort Myers Miracle, the team he played for in 1995–96 to start his baseball career.