Hemery's first International title came at the 1966 Commonwealth Games, where he won the 120 yd hurdles in 14.1 seconds, a title he retained four years later at the 1970 Commonwealth Games (by then it was the 110m hurdles which he won in 13.8 seconds).
At the Mexico City Olympics in 1968, Hemery won the 400m hurdles in 48.12 seconds, a new world record. His margin of victory was the largest since the 1924, beating second-placed Gerhard Hennige from West Germany by almost a second. BBC coverage of the race is notorious for the words of the commentator David Coleman, shortly after Hemery crossed the line: "who cares who's third – it doesn't matter!" Hemery's British teammate John Sherwood turned out to be the bronze medal winner in a close finish that had to be settled by slow motion replay. It was an early example of so-called Colemanballs. Hemery's performance helped him win the 1968 BBC Sports Personality of the Year.
David Hemery won the first ever British Superstars competition, held in 1973, registering a second victory in 1976, and was recognised as Britain's best Superstar from 1973 to 1977.
As a recently retired athlete, Hemery was free to participate in the professional Superstars contests and keep any prize money he won – unlike many other competitors (like Kjell Isaksson or Andy Ripley). Noticing the very high standard of competition within a short time of the event starting, Hemery created his own training regime, becoming adept at the gymnasium tests and canoeing and propelling himself into national fame.
After winning the original British title, Hemery then entered the 1974 event though he was struggling with illness. Eventually narrowly beaten by John Conteh, Hemery then went onto the 1975 European contest where he was beaten again, this time by Ties Kruize. When Kruize suffered serious injuries in a car crash before the final, Hemery was invited to participate and performed bravely, scoring points in every event. He was soundly beaten by Kjell Isaksson, but could still finish second. The rules for European Superstars allowed athletes to compete in "near specialist" events with a handicap, meaning that both Hemery and Isaksson were allowed to run in the 100m and Steeplechase, but only after giving the other finalists a head start. In the final 600m Steeplechase event Hemery had to make up a 100m handicap on his rivals in order to finish in overall second, and valiantly did so, but only after again falling badly at the water jump. Hitting the ground hard while challenging Isaksson for the lead, Hemery rose with a grimace of pain on his face then sprinted for the line, grabbing third. However as soon as the race was over he collapsed, with the TV cameras showing huge swelling to his injured leg – he had run the last 100m with badly torn ankle ligaments.
In 1976 Hemery won back his British title with a dominating performance. He could not qualify for the European Final this year (beaten by Isaksson again), so he then tried his hand at the American version, qualifying for the final before finishing a disappointing 11th. Because of this sojourn to the USA, Hemery was not eligible to compete in the inaugural 1977 World Final, and he did not compete again in individual Superstars again until the 1981 Challenge of the Champions, where he finished joint fourth with his great hurdling rival John Sherwood. Winning the 1983 Past Masters event enabled him to compete in the 1984 International Final (where he finished third, though 40 years old).