Léon Marchand was born on 17 May 2002 in Toulouse into a family swimming ran deep through — his father Xavier represented France twice at the Olympics in the 200m IM, and his mother Céline competed at the 1992 Barcelona Games. He left for the University of Texas at Austin to train under Bob Bowman, Michael Phelps' former coach, and the combination proved historic. At the 2023 World Championships he broke Phelps' 400m IM world record, the last individual world record Phelps still held, with Phelps himself commentating and later presenting the medal.
Paris 2024 was the moment the world understood what they were watching. Competing in front of a home crowd at La Défense Arena, Marchand won four individual gold medals, 400m IM, 200m butterfly, 200m breaststroke, 200m IM, plus a relay bronze, becoming the most decorated athlete of the entire Games. On 31 July, he won the 200m fly and 200m breast on the same evening, 114 minutes apart, the first swimmer to achieve that since 1976. Phelps called it the greatest double in the history of the sport. Marchand was chosen to extinguish the Olympic flame at the closing ceremony. He was 22 years old.
In 2025 he returned at the World Championships in Singapore on a reduced schedule, choosing only the IM events, and promptly smashed Ryan Lochte's 14-year-old 200m IM world record by over a second, going 1:52.69.