In 1999, Steffen won six titles at the European Junior Championships, and won a medal as a member of Germany’s relay team in the 4 × 200 metre freestyle at the 2000 Summer Olympics. In 2004, she asked to swim the 4×100 m relay. But after the Olympics, she took one year off swimming and concentrated on her studies, which were not finished.

At the 2006 European Championships in Budapest, Steffen clocked 53.30 in the women’s 100 m freestyle event, breaking the world record and upstaging the previous record of 53.42 set by Libby Lenton of Australia. Even though Lenton swam a time of 52.99 on 4 April 2007 during the mixed 400 metre freestyle relay, this time was not recognized by the FINA as world record, because the race is not considered to be a FINA event.

At the 2007 World Championships in Melbourne where she finished third in the 100 metre freestyle event and second in the 4×200 metre freestyle relay.

During the 2008 Summer Olympic Games in Beijing, China, Steffen won the 100-meter freestyle, catching world record-holder Libby Trickett of Australia at the last stroke. Steffen touched in 53.12 seconds, bettering her own Olympic record of 53.38 set on the leadoff leg of the 4×100 m freestyle relay. Steffen then edged out Dara Torres to win the 50-meter freestyle gold with a time of 24.06 seconds, winning by 0.01 seconds.

At the 2009 World Championships in Rome, Steffen clocked 52.07 in the 100 m freestyle event, breaking the world record and upstaging the previous record of 52.22 set by herself four days before. Two days later on 2 August 2009, Steffen won her second title in the 50 m freestyle event, breaking the world record with a time of 23.73 seconds.

Her 100-metre freestyle long course world record was broken by Cate Campbell on 2 July 2016. Her 50-metre freestyle long course world record was broken by Sarah Sjöström on 29 July 2017.