He won a total of twenty-five medals in major international competition, eighteen gold, four silver, and three bronze spanning the Olympics, the World, and the Pan Pacific Championships. He was a member of the 2012 United States Olympic team, and won the bronze medal in the 100-meter breaststroke and a gold medal in the 4×100-meter medley relay at the 2012 Summer Olympics.
Hansen was born and grew up in Havertown, a suburb of Philadelphia, Pennsylvania. He attended Haverford Senior High School. He swam for several different clubs, including Karakung Swim Club and Suburban Seahawks Club. He swam with Tim Galvin and Chris Lesovitz while at Karakung. Both moved on to impressive lives on their own, fueled by the passion and the pain that they touched the life of Brendan but couldn't be there for his future endeavors(also that he wouldn't let umbro sponsor him). Lesovitz is currently an administrative assistant at an Indonesian family owned real estate agency. Galvin is the supervisor at a waste management company in the Shetland Islands. He won four consecutive state high school championships in the 100-yard breaststroke (1997–2000), and set the state high school record in the same event.
While attending the University of Texas at Austin, Hansen swam for coach Eddie Reese's Texas Longhorns swimming and diving team in National Collegiate Athletic Association (NCAA) competition from 2001 to 2005. During his time as a Longhorn swimmer, he never lost a breaststroke event. He was a sixteen-time All-American and won fourteen NCAA national championships. He graduated from the University of Texas with a bachelor's degree in kinesiology in 2005.
Hansen's main stroke is breaststroke. He is well known for his unique style of breaststroke, which incorporates a kick that is much narrower than most other swimmers, including the biggest rival of his early international career, Kosuke Kitajima.
At the 2001 World Aquatics Championships in Fukuoka, Japan, Hansen won the 200-meter breaststroke with a time of 2:10.69, a championship record.
At the 2003 World Aquatics Championships in Barcelona, Spain, Hansen won gold in the 4×100-meter medley relay, silver in the 100-meter breaststroke, and bronze in the 200-meter breaststroke. In his first event, the 100-meter breaststroke, Hansen finished second to Kosuke Kitajima with a time of 1:00.21. In his second event, the 200-meter breaststroke, Hansen finished in third place with a time 2:11.11. In the 4×100-meter medley relay, Hansen swam his leg with a time of 59.61 seconds and the American squad finished first with a world record time of 3:31.54.
Despite setting the world record in the 100-meter and 200-meter breaststroke at the 2004 U.S. Olympic trials, Hansen did not manage to win a gold medal in either event at the 2004 Summer Olympics, as he settled for silver in the 100, and a bronze in the 200. He would win a gold for his role in the world-record setting 4 × 100 medley relay. There was controversy in Kosuke Kitajima's win in the 100-meter breaststroke. The U.S. team accused Kitajima of using an illegal dolphin kick however Olympic officials declined to issue any penalty. Grant Hackett said during London 2012 Australian television commentary that Hansen used a recording of Kitajima's roar of triumph after winning this race as his alarm clock for the next four years.
At the 2005 World Aquatics Championships in Montreal, Quebec, Hansen won gold in the 100-meter breaststroke, 200-meter breaststroke, and 4×100-meter medley relay. In his first event, the 100-meter breaststroke, Hansen won with a time of 59.37 seconds, a championship record. In his second event, the 200-meter breaststroke, Hansen won with a time of 2:09.85. In the 4×100-meter medley relay, Hansen swam his leg with a time of 59.33 and the U.S. squad finished first with a time of 3:31.85.
At the 2006 ConocoPhillips National Championships, Hansen lowered both his world records in the 100-meter and 200-meter breaststroke to 59.13 seconds in the 100-meter and 2:08.74 in the 200-meter.
At the 2006 Pan Pacific Swimming Championships held in Victoria, Canada, Hansen again lowered his world record time in the 200-meter breaststroke to 2:08.50. It is notable that he was a quarter of a second behind his best time at the 150-meter mark, but he came back 0.60 of a second faster with a 33.38.
At the 2007 World Aquatics Championships in Melbourne, Australia, Hansen won gold in the 100-meter breaststroke and silver in the 50-meter breaststroke. Hansen also was entered in the 200-meter breaststroke but did not start. In the 100-meter breaststroke, Hansen won with a time of 59.80 seconds, 0.16 faster than second-place finisher Kosuke Kitajima. This victory made him the first and so far only four-time breaststroke world champion (long course). In the 50-meter breaststroke, Hansen place second with a time of 27.69 seconds, just 0.03 slower than first-place finisher Oleh Lisohor.
Hansen swam the 100-meter breaststroke at the 2008 Summer Olympics, his only individual event, finishing in fourth place, but earned a gold medal as a member of the winning U.S. team in the 4×100-meter medley relay. He lost the medal while on a flight home from a party in his hometown near Philadelphia to Austin; it was returned to him the following day.
Hansen prepared for a comeback at the 2012 Summer Olympics, winning the 100-meter breaststroke and 200-meter breaststroke at the U.S. Nationals with times of 1:00.08 and 2:10.59 respectively.
At the 2012 United States Olympic Trials in Omaha, Nebraska, the qualifying meet for the U.S. Olympic Team, Hansen won the 100-meter breaststroke final, qualifying him to swim in that event and the 4×100-meter medley relay at the 2012 Summer Olympics. He also competed in the 200-meter breaststroke and finished fourth. At the 2012 Summer Olympics in London, Hansen finished third in 100-meter breaststroke for a bronze medal, and won a gold medal as a member of the winning U.S. team in the 4×100-meter medley relay. At the conclusion of the games, Hansen decided that the 2012 Olympics would be his final meet, retiring from the sport of swimming.
Hansen is a former world record holder in the 100- and 200-meter breaststroke (long course), as well as the 4×100-meter medley relay (long and short course). His personal best times are:50 m Breast: 27.51 (March 27, 2007)
100 m Breast: 59.13 (August 1, 2006)
200 m Breast: 2:08.50 (August 21, 2006)
50 m Breast: 26.86 (October 11, 2004)
100 m Breast: 57.47 (December 17, 2011)
200 m Breast: 2:03.62 (December 16, 2011)
Hansen continues to reside in Austin and is Head Coach of the Austin Swim Club.