Mills reached the major leagues in 1980 and went on to post a.256 batting average with one home run and 12 RBI in 106 games played for the Expos. He divided his time between Triple-A and the majors in each of those seasons, and sustained a right knee injury that ended his playing career at the age of 29. A full-time left-handed hitter and primarily a third baseman, he also saw time at first base and second. Mills became a part of major league trivia, when in 1983 he was Nolan Ryan's 3,509th career strikeout victim, lifting Ryan past Walter Johnson as the all-time strikeout leader.
Post-playing career
Mills managed eleven seasons in the minors in the Cubs, Rockies and Dodgers organizations, and also served as an advance scout for the Cubs. Mills was Terry Francona's first-base coach with the Philadelphia Phillies in 1997–2000. In 2003, Mills served as the Montreal Expos bench coach. From 2004 to 2009, Mills was teamed again with Francona when he served as the bench coach for the Boston Red Sox.
Houston Astros Manager
On October 27, 2009, Mills was named manager of the Houston Astros, replacing interim managerDave Clark. Mills holds the dubious distinction of guiding the Astros to the franchise's first ever 100-loss season, which he did in 2011, the trend continued the following season, as his Astros holding the worst record in the majors, Mills was fired on August 18, 2012 along with hitting coachMike Barnett and first base coachBobby Meacham. He was succeeded on an interim basis by Oklahoma City RedHawks manager Tony DeFrancesco. The team would go on to finish the season with 100+ losses for the second consecutive year.
Cleveland Indians
On October 31, 2012, Mills was hired as the third base coach of the Cleveland Indians, to work with Francona again. Mills was reassigned as the Indians bench coach in 2014. While coaching for the Boston Red Sox under Francona from 2004 to 2009 he was in the same position. On July 11, 2017, Mills managed the American League All-Star team while Francona dealt with a health issue. On July 5, 2020, Mills announced he would be opting out of the 2020 season due to the COVID-19 pandemic.