Growing up on a dairy farm Clyde Sefton did weight training, in addition to competing in boxing matches before becoming a successful cyclist. Sefton trained on his bike on the family farm in Camperdown, Victoria, while working as a bricklayer. He then set himself the goal of representing Australia at the Olympics, eventually qualifying for the 1972 München Games. Sefton, who was a complete unknown in the world of cycling, pulled off a surprise by winning silver in the road race, finishing just 27 seconds behind the winner Hennie Kuiper (NED). His medal turned out to be no fluke, with him going one better in the road race at the 1974 British Commonwealth Games in Christchurch to win gold.
Sefton returned to the Olympics for the 1976 Games in Montréal, this time placing 28th in the road race, finishing in the main peloton of riders who were just over two minutes behind the winner. After his first Olympic appearance, Sefton spent time in Europe, riding in the Milk Race in England and the Scottish Milk Race, winning the overall classification at the latter in 1973. He then signed for an Italian team, racing mainly in Italy before turning professional in 1977. Back home he had an amazing record at the Herald Sun Tour, winning 14 individual stages from 1978 to 1983, along with the overall race classification in 1981. That same year Sefton also became the Australian champion in the road race.