Born in Trowell, Nottinghamshire, Barker started his career at Rotherham as a junior making his debut in March 2003 against Brighton & Hove Albion.
[12] On 1 September 2008, Barker was named in the Press Association's Championship "Team of the Week" following his performance in the club's match at Southampton on 30 August.
[8][15] In the second half of the 2008–09 season, when Tony Parkes took over as caretaker manager, Barker was moved to his usual centre-back role and cemented his place in that position.
[19][20] Barker made his Derby début as an 89th-minute substitute in a 3–2 defeat to Scunthorpe United on 15 August, his start to the season being delayed by a slight hamstring strain.
His first goal of the season came in the ninth match, as he netted a crucial equaliser against Middlesbrough in first half stoppage time that spurred The Rams onto a 3–1 win.
[28] Although Derby once again struggled, Barker featured in every game bar one – which he missed due to suspension – throughout the campaign up until safety was assured on Easter Monday 2011.
With safety assured, he had surgery on a knee problem which had been an issue throughout most of his two years at Derby and had prevented him from training for the majority of the end of the 2010–11 season.
[29] As a result, Barker missed the first 16 games of the following season, returning to action in a reserve fixture against Sheffield United on 15 November 2011.
[37] Because he would miss the 2012–13 season due to his injury, Barker was not given a squad number, with Nigel Clough allocating his no.5 shirt to Jake Buxton.
[39] In February 2013. eleven months after his injury, it was reported that Barker had undergone three further operations and his rehab was stepped up by taking part in swimming sessions, Barker stated his relief at being able to exercise and step his recovery outside of Derby's training facility at Moor Farm and was again hopeful for a return to the Derby County team.
"[43] In early March 2015, it was reported that Barker was undergoing a trial with former manager Nigel Clough at Sheffield United, having been out of the game for almost three years.
[45] He made his debut later that day, coming on as a last minute substitute in Burton's 1–0 win against Barker's former club Derby County, his first competitive game in over four years.
[46] During the 2017–2018 season, Barker made only one appearance for Burton Albion, as a late substitute in a 3–1 win over Derby County on 14 April 2018.