He played college football for the Oregon State Beavers, and was selected by the Green Bay Packers in the first round of the 2003 NFL draft.

[6] Head coach Mike Sherman named Barnett the starting middle linebacker to begin his rookie season.

On September 14, 2003, Barnett collected a season-high 14 combined tackles (12 solo), deflected one pass, and made his first career interception during a 31–6 victory against the Detroit Lions in Week 2.

Barnett intercepted a pass by Lions' quarterback Joey Harrington, that was originally intended for tight end Mikhael Ricks, and returned it for a 13-yard gain during the fourth quarter.

[10] He finished his rookie season in 2003 with 112 combined tackle (86 solo), three pass deflections, three interceptions, and two sacks in 15 games and 15 starts.

The following week, he made five solo tackles and defended two passes as the Packers lost 20–17 at the Philadelphia Eagles during the NFC Divisional Round and were eliminated from the playoffs.

[12] Barnett missed the second half of the 2008 season after suffering a torn knee ligament during November 9's game against the Minnesota Vikings.