He played in MLB for the Florida Marlins, Boston Red Sox, Baltimore Orioles, and Toronto Blue Jays from 1998 through 2009.

He played baseball for University High School in West Los Angeles, which won the 3-A City title in 1988 under coach Frank Cruz, during his junior year.

[4] The next season, Millar helped lead the Cardinals to a 44–18 record, to the SBC regular-season and tournament championships, and also to a berth in the NCAA's Central I Regional on the campus of Texas A&M in College Station.

Millar's contract was purchased by the Marlins at the start of the 1998 season and he made his major league debut for Florida on April 11, 1998, against the Pittsburgh Pirates, finishing the game 1-for-2 with a walk after appearing as a pinch hitter in the sixth inning.

[9] He would appear in only one more game before spending the rest of the season in the minor leagues with the Triple-A Charlotte Knights, where he batted .326 with four home runs and 15 RBI.

During the 2003 playoffs, Millar began using the phrase "Cowboy Up", and in 2004 referred to his team as "idiots" to keep teammates loose during the stretch run to the World Series Championship.

Millar and Martinez stood on top of the home dugout and gave a toast that was the largest in history, according to the Guinness Book of World Records.

[14] During the season, he broke Rey Ordóñez's record for most games played by any non-drafted player who started his career in the Independent Leagues during the Draft era.

On February 11, 2009, Millar signed a minor league deal with the Toronto Blue Jays as a non-roster invitee.

On May 5, 2010, Millar returned to baseball when he signed a contract with the St. Paul Saints of the American Association, the same team with which he started his career.

Facing his first live pitching in seven years, Millar hit a two-run home run in the bottom of the second inning; the Saints went on to win the game, 8–6.

[25] In 2018, Millar and Chris Rose began co-hosting the audio podcast Intentional Talk: Caught Listening, produced by MLB Network.

The award is presented to the player who best exemplifies the spirit and desire of Jackie Jensen, the former Red Sox outfielder.

Millar co-hosts the MLB Network show Intentional Talk with Siera Santos and Ryan Dempster.