While in Huntsville, Hoelzer swam in the summer for Jones Valley Recreation Association, and swam for her high school, Huntsville High School. She also coached for JVRA.
Hoelzer swam collegiately for Auburn University, where she earned her degree in psychology with a minor in criminology. In 2009, Hoelzer relocated to Seattle, WA and then to Fullerton, California with coach Sean Hutchison to train at Fullerton Aquatics.
At the 2004 US Olympic Trials, Hoelzer qualified to swim the 200 m backstroke by placing first, with a time of 2:11.88. She also swam in the 100 m backstroke, but did not qualify to swim that event at the Olympics.
At the US Trials on July 1, 2008, Hoelzer qualified to swim in the 100 m backstroke at the 2008 Olympics. On July 5, 2008, Hoelzer broke her former Auburn University roommate Kirsty Coventry's world record of 2:06.39 in the 200 m backstroke with a time of 2:06.09, ultimately qualifying for her second event in the 2008 Olympics. This record was broken in Beijing by Coventry. She also qualified for the women's 4x100 medley relay.
Hoelzer won a bronze medal in the 100 m backstroke on August 12 in Beijing. She also won the silver medal in the 200 m backstroke on August 16, and the silver medal in the 4x100 m medley relay on August 17 after swimming in the qualifying heats for that event.