As a two-year starter at Florida State University, Diaz helped lead the Seminoles to two College World Series appearances, including the 1999 title game vs. the University of Miami. He was named the Sporting News man of the Year in 1998, and a First Team All-American (American Baseball Coaches Association, National Baseball College Writers' Association) in 1999.
He was drafted by the Tampa Bay Devil Rays in the 1999 Major League Baseball Draft in the 17th round.
Despite being among the Devil Rays' minor league career leaders in hits and extra base hits, Diaz had a tough time in the majors, hitting only .167 in 30 at-bats in his two years with the Devil Rays.
After being released by Tampa Bay, Diaz signed with the Kansas City Royals where he hit .281 with 10 extra base hits (including 1 home run) and 12 RBI in a backup role in 2005.
On December 19, 2005, Diaz was traded to the Atlanta Braves for minor league pitcher Ricardo Rodríguez. Diaz earned a backup outfielder spot for the Braves during spring training. Appearing in 124 games, including many starting assignments in the second half of the season, Diaz batted .327 in 297 at-bats. On August 14, 2006, Diaz had four hits for the second straight day and tied the National League record with ten straight hits. He grounded out in the 9th inning to end the streak.
Diaz had his first career two-homer game on August 29, 2007, against the Florida Marlins. On September 5, he hit a three-run walk-off double in the bottom of the ninth to give the Braves a 9-8 win. Diaz finished the season with a career-high .338 batting average in 124 games.
In December 2010, Diaz agreed to a two-year, $4.25 million contract with the Pittsburgh Pirates.
On August 31, 2011, Diaz was traded back to the Braves for a player to be named later. Pittsburgh also sent cash to the Braves in the deal. On August 13, 2012, the Braves announced that Diaz will have surgery on his thumb and will miss the rest of the 2012 season.
On December 26, 2012, Diaz signed a minor-league contract with the New York Yankees with an invitation to Spring Training. He was released on March 17, 2013.
Diaz was brought up by the Miami Marlins from their Triple-A affiliate New Orleans Zephyrs on May 2, 2013.
On Feb. 4th, 2014 Matt Diaz announced his retirement from professional baseball. On April 24, 2014 Diaz joined the Braves pre-game and postgame telecast of Braves Live on Fox Sports South, which he will do regularly for the 2014 season.. Daiz is currently on MLB Radio.
A born-again Christian, Diaz and his wife Leslee have three kids, Nathan Everett, Anna Grace, and Matthew Jacob. Matt is the second oldest of four brothers: Zachary, contemporary Christian singer Jonny Diaz, and Benjamin. His surname has an Anglicized pronunciation which originated with his grandfather who had emigrated from Barcelona. He grew up in Lakeland, Florida.On not being an everyday player: "[A] lot of people would think about the money they're not making if they don't play every day, but we all make enough money."
Matt Diaz is the center of a viral video of him getting heckled by a New York Mets fan. He was heckled for wearing two different socks, and Diaz responded by showing the fans behind him the variation in color.