Schmitt was born in Pittsburgh, Pennsylvania in 1990 to Ralph and Gail Schmitt. Raised as one of four siblings in what is still her hometown of Canton, Michigan, she went to Canton Charter Academy for elementary and middle school. Schmitt’s father is a financial analyst and her mother is a system project manager. Schmitt considers her parents to have been the most influential and helpful people in her life. “They are so supportive of my dreams and ambitions,” she said in 2008, adding, “they have given me so many opportunities to excel in life and have set a good example for me to follow.”

From ages 10 through 13, Schmitt swam with the Ann Arbor Swim Club (AASC) in Ann Arbor, Michigan (which, in 2006, merged into Club Wolverine). In an interview during the 2012 Summer Olympics, Josh Morgan, her then-AASC coach, said there was no indication in those years just how fast she would one day become. He said she started to show real potential, later, in the spring of her junior year in high school when she went to a 2007 Junior National Team competition. Calling it “her first real breakout swim,” he said she went “from pretty fast to really fast,” dropping four to five seconds off her 200-meter freestyle short course time of normally around a minute fifty-two seconds, down to around a minute forty-seven, thereby putting Schmitt in what Morgan called “elite company.”

She has an older sister named Kirsten who earned her bachelor’s degree from the University of Florida, and her law degree from Washington University School of Law in St. Louis, Missouri. Her older brother, Derek, swam for the University of Pittsburgh, and is now an assistant coach for the Arizona State Sun Devils swim team in Tempe, Arizona. Schmitt’s younger twin sisters, Kari and Sara, played in the USA Hockey national championship games in 2008 and 2009, lettered in varsity both basketball and swimming during their four years in high school, and played hockey at Ohio State University.

At the 2008 Summer Olympics, Schmitt won a bronze medal in the 4×200-meter freestyle relay with Natalie Coughlin, Caroline Burckle and Katie Hoff. Swimming the lead-off leg, Schmitt recorded a split time of 1:57.71 and the American team finished with a time of 7:46.33, an American record. Schmitt also competed in the 200-meter freestyle, but did not advance past the semifinals, finishing in ninth place with a time of 1:58.01.

After the Summer Olympics, in the fall of 2008, Schmitt moved to Athens to become a freshman at the University of Georgia, majoring in psychology, and minoring in childhood and family development. She joined coach Jack Bauerle’s Georgia Bulldogs swimming and diving team, following her competition in the Summer Olympics under Bauerle, who was the women’s swimming head coach. She participated in NCAA competition during her freshman, sophomore and junior years, ultimately becoming a four-time NCAA national champion by winning the 500-yard freestyle in 2009, 2010, and 2011, and the 200-yard freestyle in 2010. While at college in Athens, Schmitt also became involved in Athens Bulldog Swim Club (ABSC) competitions on the University of Georgia campus.

At the 2009 World Championships in Rome, Schmitt competed in three events, the 200-meter and 400-meter freestyle, and the 4×200-meter freestyle relay. In her first event, the 400-meter freestyle, Schmitt recorded the second best time in the heats with a time of 4:02.80. However, in the final she placed 4th with a time of 4:02.51. In her second event, the 200-meter freestyle, she won a silver medal finishing behind world record holder Federica Pellegrini by 1.98 seconds. Schmitt’s time of 1:54.96 was good enough for the American record. In the 4×200-meter freestyle relay, she swam the final leg in 1:54.21 as the American team placed second to China. The final time of 7:42.56 was just behind China’s time of 7:42.08, but was good enough for the American record.

At the 2011 World Aquatics Championships in Shanghai, Schmitt won a gold medal in the 4×200-meter freestyle relay with Missy Franklin, Dagny Knutson, and Katie Hoff, with the team finishing ahead of Australia and China. As the anchor leg, Schmitt had a 1:56.49 split. Schmitt also competed in the individual 200-meter freestyle and finished sixth in the final (1:56.98).

After the 2012 Summer Olympics, Schmitt returned to the University of Georgia to complete her senior year of college, She was the recipient of the Honda Sports Award for Swimming and Diving, recognizing her as the outstanding college female swimmer of 2012–13. Following her return from London, Schmitt started suffering from depression, finding the increased public attention to be overwhelming. Her anguish led to bad swimming results, leading her to not qualify for various international competitions. After opening up her feelings to Phelps and Bowman in January 2015, Schmitt decided to attend therapy sessions in secret to her family. Once Schmitt’s cousin April Bocian committed suicide in May, she opted to reveal herself to her family and also make her emotional issues public, feeling it would help others in the same situation. She has since been training under Bowman at Arizona State University, hoping to get in shape for a possible third Olympic appearance at the 2016 Summer Olympics.

At the 2012 United States Olympic Trials in Omaha, Nebraska, the U.S. qualifying meet for the 2012 Olympics, Schmitt made the U.S. Olympic team by finishing first in the 200-meter and 400-meter freestyle, and third in the 100-meter freestyle. In the 200-meter freestyle final, Schmitt broke her own American record of 1:54.96 with a time of 1:54.40.

At the 2012 Summer Olympics in London, Schmitt won a total of five medals: three gold, one silver, and one bronze. In her first event, the 4×100-meter freestyle relay, Schmitt won bronze with Missy Franklin, Jessica Hardy and Lia Neal, with the U.S. team finishing third behind the women’s relay teams from Australia and the Netherlands. Swimming the anchor leg, Schmitt had a split of 53.54 seconds and the team finished with a total time of 3:34.24, an American record. Schmitt won the first individual Olympic medal of her career, a silver, in the 400-meter freestyle, and in doing so, set the American record for the event. Her time of 4:01.77 was just 0.32 seconds behind winner Camille Muffat. In her second and last individual event, the 200-meter freestyle, Schmitt won gold while setting a new Olympic record of 1:53.61, which was also a new American record. In the race, Schmitt won by a margin of 1.97 seconds over Muffat. In the 4×200-meter freestyle relay, Schmitt earned her second gold after passing Australian Alicia Coutts and topping the field with a split of 1:54.09 as the anchor leg. Also on the winning 4×200-meter relay team were Missy Franklin, Dana Vollmer and Shannon Vreeland. In her final event, the 4×100-meter medley relay, Schmitt won gold with Missy Franklin, Rebecca Soni, and Dana Vollmer. Swimming the freestyle leg, Schmitt recorded a time of 53.25, and the American team went on to set the world record with a time of 3:52.05, bettering the Chinese-owned record of 3:52.19 set in 2009.

The 2016 United States Olympic Trials saw Schmitt finishing fourth at the 200m (1:56.72), qualifying her to the relay team. Schmitt became the ninth American female swimmer to appear in three Olympics, and only she and Elizabeth Beisel were members of each team since 2008. She was named one of the captains of the U.S. Olympic Swim Team, alongside Phelps, Beisel, Nathan Adrian, Anthony Ervin, and Cammile Adams. Schmitt earned her seventh Olympic medal by taking part in the qualifying heat of the 4 × 100 m freestyle relay, and during the finals the U.S. team won the silver. Schmitt earned her fourth gold medal, and eighth Olympic medal, by swimming in the finals of the 4 × 200 m freestyle relay with Leah Smith, Maya DiRado, and Katie Ledecky.