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.