Kevin Millar's journey in Major League Baseball began with an unconventional path, as he was not drafted out of college. Instead, he signed with the independent St. Paul Saints in 1993, where his performance caught the attention of the Florida Marlins. The Marlins purchased his contract in 1994, and he worked his way through their minor league system, making his MLB debut on April 11, 1998.

Millar played for the Marlins until 2002, primarily as a first baseman and outfielder. During his time with the team, he developed a reputation for his solid hitting and on-base skills. In 2001, he had a breakout season, batting .314 with 20 home runs and 85 RBIs, which helped establish him as a reliable offensive player.

In 2003, Millar's career took a significant turn when he was claimed off waivers by the Boston Red Sox. His charismatic personality and ability to get on base quickly made him a fan favorite in Boston. Millar played a crucial role in the Red Sox's historic 2004 season, which culminated in the team winning their first World Series title in 86 years. Known for his clubhouse presence, Millar famously rallied his teammates with the phrase "Cowboy Up" during the playoffs, and his walk in Game 4 of the ALCS against the New York Yankees sparked the Red Sox's comeback from a 3-0 series deficit.

After his tenure with the Red Sox, Millar signed with the Baltimore Orioles in 2006. He spent three seasons with the Orioles, continuing to contribute as a steady hitter and veteran presence in the clubhouse. In 2009, Millar joined the Toronto Blue Jays, where he played his final season in the majors.

Throughout his career, Millar was known for his ability to get on base, posting a career on-base percentage of .358. He finished his MLB career with a .274 batting average, 170 home runs, and 699 RBIs.