He won five Olympic gold medals and was a world champion nine times. He was the first man to win both the 200m and 400m races at the same Olympics, accomplishing the feat at the 1996 Summer Olympics.

Johnson was noted for his unique running style. His upright stance and very short steps defied the perceived wisdom that a high knee lift was essential for maximum speed.

A 1990 graduate of Baylor University, Johnson won a number of medals at the NCAA Indoor Track and Field Championships during his collegiate career. Johnson's breakthrough onto the world stage came in 1991 when Johnson won the World Championship 200m title in Tokyo.

A month before the 1992 Summer Olympics Johnson contracted food poisoning, which caused him to become weight drained and fall out of peak condition, and in turn, fail to qualify for the 200m final. He was able to race as a member of the 4 x 400m relay team, which won a gold medal and set a new world record (2:55.74).

Johnson won the 400m at the 1993 World Championships in Stuttgart. He also won another gold medal as a member of the 4 x 400m relay team, setting a new world record (2:54.29). In the next World Championships at Gothenburg Johnson did his first "double" in a major tournament, winning both the 200m and 400m.

The next year Johnson broke the 200m world record when he ran 19.66 s at the USA National Championships. With that performance he qualified to run at the 1996 Olympic Games in Atlanta and prepared to attempt what some considered impossible — to win both the 200m and 400m races, a feat no man had ever achieved. (Two women have won Olympic gold medals in both races in the same year: Valerie Brisco-Hooks in 1984 and Marie-José Perec in 1996.)

On July 29 he easily captured the 400m Olympic title with a time of 43.49 s, beating Roger Black of Great Britain by almost a second. At the 200m final on August 1, Johnson ran the opening 100m in 10.12 s, achieved a peak speed of over 40 km/h, and finished the race in 19.32 s, setting a new world record. This was the largest improvement ever on a 200m world record. Some commentators compared the performance to Bob Beamon's 1968 long jump in Mexico.

In June of 1997, he raced against Canadian Donovan Bailey in a 150m race at the Rogers Centre (then SkyDome) in Toronto. The 150m consisted of 75m of the curving track and 75m of the straight track. The race was billed as a way to determine who could legitimately claim to be the "world's fastest man." Johnson pulled out a few moments into the race with an injured quadricep, allowing Bailey to easily win the race (and the $1.5 million prize). It seemed that Donovan Bailey would have won the race regardless of Johnson's injury as he was leading after the first 75 meters and was approaching the straight line sprint to the finish line. Since the straight line sprint was Bailey's specialty, it was highly unlikely that Johnson would've won the race even if the injury had not occurred. Johnson recovered from the injury, and that same year he won his third 400m World Champion title.

In 1999 he was again plagued by injury, and his following season was troubled with two injury scares that limited him to just four 400m races before the World Championships in Seville. Were it not for the IAAF's policy of allowing defending champions automatic entry, he would not have raced in Seville, since he failed to compete in the USA trials due to his injury. He recovered and won his fourth 400m World Champion title with a new world record of 43.18 s. He later ran the last leg of the 4 x 400m relay, adding a ninth World Championship gold medal to his collection.

Johnson ended his career at the 2000 Summer Olympics in Sydney by winning the 400m gold and 4 x 400m relay race, bringing his total number of Olympic gold medals to five.

Johnson has run 22 400-meter races under 44 seconds. In the 200m, he has six times under 19.80 s and 17 sub-20 second performances. He also holds the world record at 300m — 30.85 s. The former record was 31.48 s.