As a youth, Barker played in the 2000 Quebec International Pee-Wee Hockey Tournament with the Winnipeg South Monarchs minor ice hockey team. He later played AAA Midget Hockey for the Winnipeg Thrashers at age 14. After that season he was drafted 4th overall in the 2001 WHL Bantam Draft by the Medicine Hat Tigers.

Barker began his junior hockey career in 2001–02 with the Medicine Hat Tigers of the Western Hockey League (WHL), appearing in three games. The following season, he led his team in scoring among defencemen with 47 points and was named to the WHL All-Rookie Team. He improved to a junior career-high 65 points in 60 games in 2003–04, leading the Tigers to a President’s Cup championship. As WHL champions, they earned a berth in the 2004 Memorial Cup, however, Medicine Hat was defeated by the Gatineau Olympiques in the semi-final.

He was drafted third overall by the Chicago Blackhawks in the 2004 NHL Entry Draft and was signed to an entry-level contract by Chicago on August 22, 2005. He made the Blackhawks opening roster out of training camp in 2005–06 and made his NHL debut against the Colorado Avalanche, but was returned after one game to Medicine Hat, where he completed the season with 18 points in 26 games.

Barker began the 2006–07 season injured, having undergone ankle surgery in September. After recovering, he joined the Norfolk Admirals, Chicago’s American Hockey League (AHL) affiliate, making his Norfolk debut on November 3, 2006. He recorded his first professional point on November 21 against the Manchester Monarchs, assisting on the game-winning overtime goal. A few games later, he scored his first professional goal on November 29 against the Bridgeport Sound Tigers in a 6–3 loss. Later in the season, he was called up by the Blackhawks and scored his first NHL goal on January 14, 2007 against the Minnesota Wild. He finished his rookie season playing in 35 games with Chicago and recording 8 points.

Barker was a part of Team Canada’s gold medal winning team at the 2005 World Junior Championships. However, he contracted mononucleosis midway through the tournament and was only able to appear in the first 3 games. Barker made his second straight World Junior appearance in 2006 as the only returning player on Team Canada’s roster. He repeated as gold medal champion with Team Canada, which defeated Russia in the final.

Despite finishing his rookie season with the Blackhawks, he was reassigned to the AHL to begin 2007–08. Joining the Rockford IceHogs in their inaugural AHL season (the team previously played in the United Hockey League), he assisted on the first ever goal in IceHogs AHL history on October 6, 2007 against the Quad City Flames. Barker had been selected to play at the 2008 AHL All-Star Classic, but did not attend as he was once again recalled by the Blackhawks. In his second stint with the Blackhawks, he improved to six goals and 18 points in 45 games.

With Barker becoming a restricted free agent in the off-season, the NHL Players Association filed a grievance on July 6, 2009, against the Blackhawks on Barker’s behalf for not extending a qualifying offer by the required deadline. As such, the NHLPA argued that Barker (as well as five other Blackhawks players in the same situation) qualified as an unrestricted free agent and could entertain offers from other teams around the league. The Blackhawks quickly came to terms with Barker the same day, re-signing him to a three-year contract worth US$9.25 million.

On February 12, 2010, Barker was traded to the Minnesota Wild for defenceman Kim Johnsson and defensive prospect Nick Leddy. He was placed on waivers by Minnesota on June 28, 2011, with the intention of buying out his contract.

Barker signed a one-year contract worth $2.25 million with the Edmonton Oilers on July 1, 2011. On June 25, 2012, he was not tendered a contract and released as a free agent by the Edmonton Oilers.

On September 2012, Barker was invited to the AHL’s Texas Stars training camp and later made it to the main roster. Barker marked his first return to the AHL since 2008 in debuting for the Stars to start the 2012–13 season, on October 13 in a game against San Antonio Rampage. After 23 games with Texas, his professional try-out was not extended and he left the team.

On January 13, 2013, Barker signed a one-year contract worth $700,000 with the Vancouver Canucks. Barker was not re-signed by the Canucks and became an unrestricted free agent on July 5, 2013.

On November 5, 2013, Barker signed a one-year deal with Barys Astana of the Kontinental Hockey League (KHL). After spending the 2013–14 season in the KHL, Barker returned to North America and attended training camp with the Blackhawks on a player tryout offer. However, on September 27, 2014, the Blackhawks announced that they had released Barker, leaving him as an unrestricted free agent. On December 22, 2014, Barker signed a contract until the end of the 2014–15 KHL season with HC Slovan Bratislava, and then had his contract renewed for the 2015–16 season. He left Bratislava after two years to return to Barys Astana in May 2016.

After another shortened stint with Slovan Bratislava, Barker left the KHL and signed with the SCL Tigers of the Swiss National League (NL) on January 12, 2018, until the end of the 2017-18 season. In November 2018 he signed a contract until the end of season (including a try-out until the end of November) with Ilves Tampere but after playing seven games, his contract was terminated on try-out.