At the 2012 United States Olympic Trials in Omaha, Nebraska (her first senior national competition), Ledecky made the Olympic team by placing first in the 800-meter freestyle with a time of 8:19.78, which was over two seconds ahead of second-place finisher Kate Ziegler. In Omaha, Ledecky also placed third in the 400-meter freestyle (4:05.00) and ninth in the 200-meter freestyle (1:58.66). Her third-place finish in the 400-meter freestyle was the fastest time ever swum by a 15- to 16-year-old American. At 15 years, 4 months, and 10 days, she was the youngest American participant at the 2012 Olympic Games.

At the 2012 Summer Olympics in London, Ledecky qualified to swim in the final of the 800-meter freestyle by placing third overall in the heats with a time of 8:23.84. In the final, Ledecky stunned the field, winning gold by more than four seconds, with a time of 8:14.63, the then second-fastest effort of all time just behind Rebecca Adlington’s world record of 8:14.10 set in 2008. In addition, she broke Janet Evans’ American record of 8:16.22 that had stood since 1989. In the final, Ledecky went out hard and, by the 200-meter mark, she had already established an almost body-length lead. Her 400-meter split was 4:04.34, a personal best for Ledecky in that distance, and would have placed fifth in the individual 400-meter freestyle. At the 750-meter mark, Ledecky was 3.42 seconds ahead of Mireia Belmonte García, and 0.31 seconds under world record pace. Ledecky won by 4.13 seconds and just missed the world record by 0.53 seconds. Her gold was the first international medal of her career, earning her the 2012 Best Female Performance of the Year and Breakout Performer of the Year at the Golden Goggle Awards.

Ledecky was a USA Olympic team member in 2012 and 2016, and holds the record for most individual gold medals (11) and overall gold medals (15) at the World Aquatics Championships.

At the 2013 US National Championships, Ledecky qualified to swim in four individual events and the 4×200-meter freestyle relay at the 2013 World Aquatics Championships in Barcelona, Spain, though she later dropped the 200-meter freestyle from her program. At the National Championships, she finished first in the 400-, 800- and 1500-meter freestyle, and second in the 200-meter freestyle.

At the 2013 World Championships, Ledecky won gold in the 400-, 800-, and 1500-meter freestyle, and in the 4×200-meter freestyle relay, and set two world records. In winning the 400 through 1500-meter titles, she became the second woman ever to win the events in a World Championships since German Hannah Stockbauer in 2003. In her first event in Barcelona, the 400-meter freestyle, Ledecky became a world champion for the first time by winning in 3:59.82, setting a new American record and becoming the second-fastest performer of all time in the event.

At the 2014 US National Championships, the qualifying meet for both the 2014 Pan Pacific Swimming Championships and the 2015 World Aquatics Championships, Ledecky finished first in the 200-, 400-, and 800-meter freestyle. In the 400-meter freestyle, Ledecky set her third world record of the year by breaking Federica Pellegrini’s 2009 world record of 3:59.15 with a time of 3:58.86. With her mark in the 400, Ledecky became the first female since Janet Evans to hold world records simultaneously in the 400-, 800-, and 1500-meter freestyles.

At the 2014 Pan Pacific Swimming Championships in Gold Coast, Australia, Ledecky won five gold medals and broke two world records. Her wins came in the 200-, 400-, 800-, and 1500-meter freestyle and the 4×200-meter freestyle. Ledecky almost broke the world record in the 800-meter freestyle, finishing with a time of 8:11.35. Her world records came in the 400- and 1500-meter freestyles, with times of 3:58.37 and 15:28.36, respectively. She became the first woman to win four individual gold medals at a single Pan Pacific Championship.

Ledecky began swimming at the age of six, due to the influence of her older brother, Michael, and her mother, who swam for the University of New Mexico. In Bethesda, she attended Little Flower School through eighth grade and graduated from Stone Ridge School of the Sacred Heart in 2015. During her high school swimming career, Ledecky twice set the American and US Open record in the 500-yard freestyle and twice set the national, high-school record in the 200-yard freestyle. Ledecky finished her high-school career as the holder of the Stone Ridge school record in every swimming event except the 100-meter breaststroke.

At the 2015 World Aquatics Championships in Kazan, Russia, Ledecky won five gold medals and broke three world records. Her wins came in the 200-, 400-, 800-, and 1500-meter freestyles and the 4×200-meter freestyle. Her world records came in the 800- and 1500-meter freestyles. Ledecky is the first swimmer to win the 200-, 400-, 800- and 1500-meter freestyles in a major competition. Ledecky was also named the female swimmer of the meet.

In December 2016, Ledecky was chosen as one of the sponsors of the US Navy aircraft carrier USS Enterprise alongside gymnast Simone Biles. They are the first Olympians to be given this honor.

Ledecky began 2016 at the Arena Pro Swim Series at Austin. On the final day of competition, she reset her world record in the 800-meter freestyle, clocking a time of 8:06.68. Earlier in the meet, she set world-leading times in the 200- and 400-meter freestyles and a U.S.-leading time in the 100-meter freestyle.

At the 2016 United States Olympic Trials in Omaha, Nebraska, Ledecky won the 200-, 400- and 800-meter freestyles to qualify for her second Olympic team. Ledecky opened the Trials with a win and meet record in the 400-meter freestyle (3:58.98). Two days later, she recorded a time of 1:54.88 en route to winning the 200-meter freestyle. On July 1, Ledecky broke Janet Evans’s 28-year-old national championship record in the morning preliminary heats of the 800-meter freestyle (8:10.91) before placing seventh that evening in the 100-meter freestyle (53.99). The next day, Ledecky lowered her meet record, winning the 800-meter freestyle final by nearly 10 seconds (8:10.32). With her pair of swims in the 800-meter freestyle, Ledecky took control of the top 11 fastest times in the event’s history. At the conclusion of the week, Ledecky was named the female swimmer of the meet.

Ledecky’s first event in the 2016 Summer Olympics was the 4×100-meter freestyle relay, swimming the anchor leg for the United States in both the prelims and final. Ledecky recorded a split of 52.64 in the heats. In the final, she joined Simone Manuel, Abbey Weitzeil, and Dana Vollmer and recorded a split of 52.79, helping the team earn a silver medal (behind Australia) with a time of 3:31.89, a new national record for the United States.

At the Ohio State Invitational in November 2016, Ledecky lowered her American and U.S. Open marks in the women’s 500-yard freestyle and 1650-yard freestyle.

At the 2017 US National Championships, the qualifying meet for the 2017 World Aquatics Championships, Ledecky won the 200-, 400-, and 800-meter freestyle and earned a spot on six events.

At the Texas A&M Invitational in November 2017, Ledecky lowered her American and U.S. Open mark in the 1650-yard freestyle with a time of 15:03.31.

Ledecky made her debut as a professional swimmer on May 16, 2018, at the TYR Pro Swim Series in Indianapolis, Indiana, where she beat her old world record in the 1500-meter freestyle, setting a new world record of 15:20.48. She broke the existing mark by five seconds.

Ledecky signed her first sponsorship deal with TYR Sport, Inc. on June 8, 2018.

In July 2018, at the US National Swimming Championships, Ledecky qualified for the 2018 Pan Pacific Championships by finishing first in the 200-, 400-, and 800-meter freestyle.

At the 2018 Pan Pacific Championships in Tokyo, Ledecky led all swimmers with three individual titles, winning golds in the 400-, 800-, and 1500-meter freestyle. She also took bronze in the 200-meter freestyle and silver in the 4×200-meter freestyle.

At the 2019 World Aquatics Championships in Gwangju, South Korea, Ledecky battled illness and withdrew from two events before winning her fourth-straight World Championship title in the 800-meter freestyle.