Drafted by the Florida Marlins in 1993, McMillon debuted in the major leagues for the Marlins, where he played for parts of and and batted.188 in 41 games played before being traded to the Philadelphia Phillies for Darren Daulton on July 21, 1997. He also played in the major leagues for the Oakland Athletics and Detroit Tigers. He appeared in 269 MLB games, and his 149 big-league hits included 35 doubles, three triples and 16 home runs. He hit.301 for the 2000 Tigers, but the season, which he split between Detroit and Oakland, was his only full season in the majors.
Managing and coaching career
McMillon joined the Boston organization as batting coach of the Single-AGreenville Drive of the South Atlantic League in 2008–09, then was promoted to manager of the Drive in 2010. In two seasons, he led Greenville to a 155–124 record and one playoff appearance. Then, on January 20, 2012, he was promoted to manager of the Salem Red Sox of the Class ACarolina League. After finishing one game under.500 in 2012, he was rehired as Salem's manager for the 2013 season and led his team to the Carolina League championship. Salem won the second half Southern Division title, and then bested the Myrtle Beach Pelicans and the Potomac Nationals in the playoffs. The Salem Red Sox won their final 11 games during the regular season and playoffs. On December 18, 2013, McMillon was promoted to manager of the Portland Sea Dogs, the Red Sox' affiliate in the Double-AEastern League, for 2014. McMillon returned to Portland as a manager 19 years after starring for the 1995 Sea Dogs, then affiliated with the Marlins. That season, the 23-year-old McMillon batted.313 for Portland, with 14 home runs and 93 runs batted in. He led the Eastern League in hits and was named an outfielder on the EL All-Star team as well as the Marlins' minor-league player of the year. His managerial debut with the 2014 Sea Dogs produced a first-place finish in the Eastern League's Northern Division with an 88–54 record. McMillon was named the league's manager of the year. During the season, his club included top prospects such as Mookie Betts, Blake Swihart, Henry Owens, Deven Marrero, Brian Johnson, Eduardo Rodríguez and Travis Shaw; all but Rodríguez were promoted to higher levels by the time of the EL playoffs, when Portland fell to the Binghamton Mets in the first round. McMillon was rehired by the Sea Dogs as their 2015 manager on January 8. McMillon spent 2016–18 as the roving minor league outfield and base running coordinator in the Red Sox' farm system. In returning to managing in 2019, his 12th year in the organization, McMillon becomes the 18th manager in the PawSox' Triple-A history and 21st overall since the team was established as a Double-A franchise in 1970. Through 2015, McMillon's six-year, regular season won-lost record as a manager is 440–400. In 2019, the International League announced that McMillon would be inducted into its hall of fame.