In his ten-year Major League Baseball (MLB) career, Bellhorn was best known for being the starting second baseman for the Boston Red Sox during their 2004 World Series championship season.

Mark Bellhorn has one brother, Todd, who played in the New York Mets minor league system in 1998 and 1999.

[3] After playing college ball at Auburn, Bellhorn broke into the majors with the Oakland Athletics in 1997, drafted in the second round.

[4] On June 20, 2003, he was traded to the Colorado Rockies, and finished the year hitting .221 with two home runs and 26 RBI.

In 2004, Bellhorn was signed by the Boston Red Sox as a utility infielder; however, he became the regular second baseman after Pokey Reese and Nomar Garciaparra suffered early-season injuries.

Bellhorn then hit a three-run homer off Jon Lieber to give the Red Sox a 4–2 victory over the Yankees in Game 6.

He also homered in Game 7 of the ALCS, which the Red Sox went on to win, sending the ball high and clanging it off the right-field foul pole.

[8] In 2006, he joined the San Diego Padres, playing in 115 games but hitting .190 for his batting average for the season.

[10] On August 12, 2007, the Reds designated Bellhorn for assignment to make room for Josh Hamilton, who was coming off the 15-day disabled list.