Millar signed a free agent contract with the Marlins in 1993. He spent the next five seasons between several minor league teams for Florida, making his Major League debut with the team in 1998, playing in two games.
After 36 games at Marlins' AAA affiliate Calgary, Millar returned to the parent club in May of 1999. He made the most of his opportunity, hitting .285 with 67 RBI over the next 105 games.
Millar remained with the club in 2000, hitting .259 over 123 games. 2001 was his best season with the Marlins. He hit .314 with 20 home runs and 85 RBI. Over his five seasons, Millar played first base, third base, and left field as needed. He totalled 59 home runs and 251 RBI in 500 games, hitting .296.
In 2003, Millar's contract was bought by the Boston Red Sox. He later also played for the Baltimore Orioles and the Toronto Blue Jays.
He is currently an analyst with NESN and the MLB network.