Phelps was raised in a family of swimmers and joined the prestigious North Baltimore Aquatic Club at age seven. He finished fifth in the 200-metre butterfly at the 2000 Olympic Games in Sydney, Australia. At the 2001 U.S. spring nationals he became, at age 15, the youngest world-record holder in men's swimming when he posted 1 min 54.92 sec in the 200-metre butterfly. He went on that year to win his first international title at the world championships in Fukuoka, Japan. He claimed five medals at the 2002 Pan Pacific championships, including three gold (200-metre and 400-metre individual medley and 4 × 100-metre medley relay). At the U.S. spring nationals in 2003, he became the first male swimmer to claim titles in three different strokes at a single national championship, and he later broke an unprecedented five individual world records at the world championships in Barcelona, Spain. Phelps also captured five titles at the U.S. summer nationals—the most won by a male swimmer at a single championship.

Phelps entered the 2008 Olympic Games in Beijing with the goal of breaking Spitz's record of seven gold medals at one Olympics. He took the gold in each of his first three events—the 400-metre individual medley, the 4 ×100-metre freestyle relay, and the 200-metre freestyle—and each victory took place in world record time. On August 13 he won golds in the 200-metre butterfly and the 4 × 200-metre freestyle relay to capture his 10th and 11th career gold medals, a new Olympic record. Phelps then won his sixth gold of the Beijing Games by breaking his own world record in the 200-metre individual medley. He tied Spitz's record by winning the 100-metre butterfly final by 0.01 second and broke the mark as a member of the victorious American 4 × 100-metre medley relay team. All told, Phelps set world records in all but one (the 100-metre butterfly) of his eight gold-medal-winning events.