James was drafted in the fourth round of the 2003 NFL Draft by the Dallas Cowboys. He was the sixth middle/inside linebacker taken, behind Nick Barnett of Oregon State, Kawika Mitchell of South Florida, Gerald Hayes of Pittsburgh, Cie Grant of Ohio State, and Angelo Crowell of Virginia.
He backed up Pro Bowl outside linebacker Dexter Coakley in a 43 defense during his first two seasons in the league, while showing playmaking ability as a special teams performer in 2003 and 2004.
In 2005, the Cowboys changed to a 34 defense, allowing him to have a breakout year, while making the transition from special teams player to starting middle linebacker to lead the Cowboys with 109 tackles.
In 2008, James became only the second Cowboys defender (Eugene Lockhart - 222 in 1989) to reach 200 tackles in a season.
James was the first player to lead the Cowboys in tackles more than three straight seasons. In 2009 he led the team in tackles for the fifth consecutive season, the longest streak in franchise history.
During his career, he also had to deal with the uncertainty of who would start at the other inside linebacker position, having to play with different teammates who included: Scott Shanle, Akin Ayodele, Zach Thomas, Keith Brooking and Sean Lee.
In 2011 with the emergence of Lee, James and Brooking split snaps at the other inside linebacker spot, which freed James to return kickoffs for the first time since 2005.