Barker reached a world ranking of 13 in November 2007, the same month that he was selected to represent the senior England team at the World Team Championships in December 2008, held in India.
Peter Barker has 12 professional tour titles to his name out of 15 final appearances. In September 2008, Barker defeated David Palmer twice, with a decisive 3–0 victory over David Palmer in Chicago at the Sweet Home Chicago Open, and a 3–2 victory in Baltimore at the Merrit Properties Open. Barker's highest world ranking of 7 was reached in May 2009.
In 2010, Barker won the bronze medal in the 2010 Commonwealth Games squash men's singles event in Delhi after defeating Mohd Azlan Iskandar from Malaysia in straight sets, 11-5, 11–4, 11–2. The match ended in 45 minutes. Barker had match point against Ramy Ashour in the quarterfinals of the 2012 U.S. Open, but lost the match. Ashour went on to win the tournament.