Barnett was a four year letter winner (1999–2002) at Oregon State, starting the last three seasons at strong side linebacker. As a senior, Barnett was a first team All-Pacific-10 Conference selection. He led the conference in tackles with 121 (62 solo). Barnett registered his single game best against the University of California in 2001 with 18 tackles (11 solo).
He majored in Business Administration and Communications.
On April 10, 2007, Barnett signed a six year contract extension worth $34.85 million.
Barnett missed much of the 2008 season after suffering a torn knee ligament during November 9's game against the Minnesota Vikings.
Barnett suffered a season-ending wrist injury in a Week 4 matchup vs. the Detroit Lions. He was put on injured reserve on October 7, 2010. It's the second time in the last three seasons Barnett ended his season on injured reserve. Barnett is second in all-time tackles for the Green Bay Packers.
On July 26, 2011, Barnett was informed by Packers General Manager, Ted Thompson, that the Packers will try to trade him, but if not, they will release him. He was released on July 28, after Green Bay was unable to find a trade.
Barnett signed a three year, 12 million dollar deal with the Buffalo Bills on July 31, 2011. On February 11, 2013 the Bill's announced that Barnett will be released from his contract along with SS George Wilson.