After a brief loan spell with non-League side Aylesbury United, he established himself in the first team at Luton and made 70 appearances in all competitions before signing for West Bromwich Albion in 2007 for an initial fee of £2.5 million.

[5] He signed a professional contract on 27 September 2004,[6] one day before making a starting appearance in a 2–0 away defeat to Swansea City in the Football League Trophy, during which he was shown a red card after only 12 minutes for a foul on Adrian Forbes.

[12] Barnett began the 2005–06 season by making his Football League debut as a 90th-minute substitute for Russell Perrett in a 1–0 away win over Hull City on 17 September 2005.

[13] Three days later, he made a starting appearance in a 1–0 away defeat to Reading in the League Cup second round, but was substituted in the 55th minute and replaced by Keith Keane.

[15] He featured as a 74th-minute substitute for Markus Heikkinen as Luton were defeated 5–3 at home to Premier League club Liverpool in the FA Cup third round, a match they led 3–1.

[26] Barnett completed a move to Championship club West Bromwich Albion on 26 July 2007 on a four-year contract for an initial fee of £2.5 million.

[36] He played the full 90 minutes in a 0–0 draw with Bolton Wanderers four days later, helping to keep a clean sheet to earn West Brom their first point of the season.

[44] Barnett's injury meant he missed a 2–2 draw with Burnley one week later,[45] before returning to the starting lineup for the next match, a 3–2 home defeat to Newcastle United.

[48] On 4 November 2009, Barnett joined Championship rivals Coventry City on loan until the end of January,[49] having made just two substitute appearances for West Brom in the league during the 2009–10 season.

[50] On 26 August 2010, he joined Norwich City on loan until 4 January 2011, to cover for injuries to Michael Nelson and Zak Whitbread,[54] after having played his first match of the 2010–11 season for West Brom two days previously in a 2–0 away win over Leyton Orient in the League Cup second round.

[55] He was handed squad number 20 at Norwich and made his debut two days later in a 1–1 draw with Nottingham Forest at the City Ground, forming a partnership with Elliott Ward.

[62] Barnett played in his first Premier League match for Norwich as a 71st-minute substitute for the injured Zak Whitbread in a 1–1 draw with Wigan Athletic on the opening day of the 2011–12 season.

[63] He started the next game, a 1–1 draw with Stoke City, but was adjudged by referee Neil Swarbrick to have committed a professional foul on Jonathan Walters in the second half, conceding a penalty and receiving a red card.

[65] After featuring in a 2–1 home defeat to Leicester City in the FA Cup fifth round on 18 February 2012,[66] Barnett was only named in the matchday squad on two further occasions, but remained an unused substitute in both matches.

[70] On transfer deadline day, it appeared Barnett was no longer in the future plans of manager Chris Hughton, after he was offered as part of a bid in an attempt to sign Birmingham City defender Curtis Davies.

[71] On 14 March 2013, Barnett completed a one-month loan move to Championship club Cardiff City as a replacement for injured captain Mark Hudson.

[74] He formed a defensive partnership with Ben Turner during his spell at the club, making eight appearances,[69] as Cardiff won the Championship title and promotion to the Premier League.

[76] On 1 August 2013, Barnett signed a three-year contract with newly relegated Championship club Wigan Athletic, transferring from Norwich City for an undisclosed fee.

[82] Wigan secured a place in the Championship play-offs after a 1–0 away win over Birmingham City on 29 April 2014, during which Barnett replaced Shaun Maloney as a 90th-minute substitute.

[83] A 2–1 defeat to Queens Park Rangers in the play-off semi-final second leg ended Wigan's hopes of an instant return to the Premier League after a 96th-minute winning goal from Charlie Austin, before which Barnett replaced Gary Caldwell as a 74th-minute substitute.

[89] Barnett played every minute of Wigan's matches in October,[90] before receiving a red card for tripping over Hugo Rodallega in the penalty area in a 3–3 draw with Fulham on 1 November.

[91] After serving his suspension, Barnett returned to the starting lineup and was an ever-present in the team,[90] up to and including a 2–1 home defeat to Rotherham United on 20 December.

[117] He subsequently announced his retirement with Northampton honouring the remainder of his contract and offering him the opportunity to work at the club to gain coaching experience.