Hersey was born in Athens, Georgia. Hersey is the daughter of Brian and Regina Hersey, who adopted her when she was three days old. Regina Hersey, a former freelance editor, died in January 2012 after a battle with colon cancer. Hersey says she draws inspiration from her father, a financial consultant, who has used a wheelchair since the 1980s after a spinal cord aneurysm, but still maintains an active lifestyle. She is a 2008 graduate of Marist School in Atlanta, where she set national high school records in the 100-yard butterfly and 200-yard individual medley.
Hersey competed in the 2007 Pan American Games, where she won gold medals in the 100-meter butterfly, 200-meter butterfly, 400-meter individual medley, and 4×100-meter medley relay. Also in 2007, at the U.S. Short Course Nationals, Hersey won the 400-meter individual medley. and placed second in the 200 butterfly and 200 individual medley.
Hersey qualified for the 2008 Olympic Games by placing second in the 200-meter butterfly, her signature event, at the U.S. Olympic Team Trials in Omaha, Nebraska. Additionally, she placed fifth in the final of the 400 m individual medley in Omaha with a career best time of 4:42.07. At the Olympics, Hersey finished eighth in the 200-meter butterfly finals with a time of 2:08.23. She had swum a then career best 2:06.96 in the semi-finals of this event a day earlier.
At the 2009 USA Swimming National Championships, Hersey won the 200-meter butterfly in a new career best time of 2:06.44, earning a place on the 2009 World Championships roster. Hersey also placed fifth in the 100 m butterfly in 58.36. At the 2009 World Aquatics Championships in Rome, Hersey placed ninth overall in 2:06.89, barely missing a place in the final.
At the 2012 United States Olympic Trials in Omaha, Nebraska, the U.S. qualifying meet for the Olympics, Hersey made the U.S. Olympic team for the second time by finishing second in the 200-meter butterfly behind Cammile Adams with a time of 2:07.72. Hersey also competed in the 200-meter freestyle but did not advance past the semi-finals, and placed third in the 100-meter butterfly final.