Being a native of Malmö and beginning his career in Malmö Redhawks, at the time regularly oscillating between Elitserien (SHL) and HockeyAllsvenskan, Hersley was drafted by the Los Angeles Kings in the fifth round as the 139th player in the 2005 NHL Entry Draft. Hersley was traded from the Kings to the Philadelphia Flyers, along with Ned Lukacevic, in exchange for Denis Gauthier and a second round choice in the 2010 NHL Entry Draft on July 1, 2008. On September 23, 2009, Hersley was traded by the Flyers to the Nashville Predators for future considerations.
After two years in the North American ice hockey scene and another two back with the Redhawks in Malmö, as well as a brief stint with Modo Hockey, Hersley was signed to a two-year extension contract during the 2011–12 season with Leksands IF on March 13, 2012.
While having his true breakthrough season in 2013–14, Hersley scored his 22nd goal of the season on February 8, 2014, setting a new record for the number of goals by a defenceman in an SHL regular season. He also scored a hat trick while playing against Örebro HK. The previous record was set by Jan Huokko, also for Leksands IF, in the 1998–99 season with 21 goals. Hersley added another two goals to finish the regular season with 24 goals. He was awarded the Salming Trophy, as the best defenceman in the SHL, for the 2013–14 season.