Raised in Granbury, Texas, Vollmer swam for the Fort Worth Area Swim Team (FAST) coached by Ron Forrest. At the age of 12, Vollmer was the youngest swimmer to compete at the U.S. Olympic Trials. Although she did not make the 2000 Olympic team, she did qualify for the 2001 Goodwill Games, earning a bronze medal in the 4x100 medley relay. At the 2004 Olympics her 4x200 free relay set the world record, earning a gold medal. Eight years later at the 2012 Olympics, Vollmer set the world record on her way to the gold medal in the 100 butterfly and won gold in the 4x100 medley relay and 4x200 relay. She has won a total of 32 medals in international competitions, including 19 gold, eight silver and five bronze. After heart surgery in 2003 she has been an avid ambassador of the American Heart Association. Vollmer is married to Andy Grant, a former Stanford swimmer. She continues to encourage young Texas swimmers during clinics throughout the state.