[1] Barnes attended Del Campo High School in Fair Oaks, California, where he was a letterman in football and basketball.

Barnes was selected with the 46th overall pick by the Memphis Grizzlies in the 2002 NBA draft, and was immediately traded along with Nick Anderson, to the Cleveland Cavaliers for Wesley Person.

Barnes signed with the Long Beach Jam of the ABA for their inaugural season in 2003, where he played with Dennis Rodman.

He made his debut for the Kings recording 17 points and 9 rebounds, establishing himself as a key rotation player for Sacramento.

[8] Barnes was traded halfway through the 2004–05 season along with Chris Webber to the Philadelphia 76ers, in exchange for Kenny Thomas, Corliss Williamson and Brian Skinner.

Although he appeared to secure a starting job with an impressive preseason and Allan Houston's retirement,[9] Barnes was waived by the Knicks after playing just six games.

Barnes signed with the Golden State Warriors before their first day of training camp in October 2006,[11] and effectively raised his status in the league since joining the team.

[15] Barnes, along with teammates Stephen Jackson and Baron Davis, served as a Warriors team captain for the 2007–08 season.

[18][19] On November 14, 2008, he received a two-game suspension for his role in the scuffle with Houston's Rafer Alston two days earlier.

[22] After the Magic were eliminated in the 2010 NBA playoffs by the Boston Celtics, Barnes announced he would opt out of the final year of his contract.

[32] On June 15, 2015, Barnes was traded, along with Spencer Hawes, to the Charlotte Hornets in exchange for shooting guard Lance Stephenson.

[35] On January 13, 2016, the NBA Players Association filed a grievance on behalf of Barnes to get his two-game suspension without pay rescinded.

[38] On March 11, Barnes recorded his first career triple-double with 26 points, 11 rebounds, and 10 assists in a 121–114 overtime victory over the New Orleans Pelicans.

Barnes was arrested in 2010 on suspicion of domestic violence against Govan, and for driving with a suspended license and threatening a police officer in July 2012.