A keen athlete as a youth, John was involved in gymnastics, rugby union and boxing. At the age of 18, he was on holiday at a Butlins holiday camp when he fell 18 metres from a big wheel resulting in the paralysis of his legs. The injury left him hospitalised for five months, and after his release he “wasted the next three years in the pub”.
Harris’ involvement in parasports began when a friend persuaded him to attend a gym in an attempt to improve his fitness and give him focus. Harris then joined a paraplegic sports club and showed promise as a competitor after winning multiple events at a local sport event. His progress was noticed and the next year he was selected for the 1980 Summer Paralympics in Arnhem in the Netherlands. Harris entered three events, the category 5 discus and shot put and the 85 kg Light-heavyweight weightlifting, but failed to medal.
Four years later Harris was back in the Great Britain team at the 1984 Summer Paralympics. Harris originally decided to not apply for the team, after being disgusted at what he saw was the poor treatment of Paralympic athletes in comparison to their Olympic counterparts. He believed that Atlanta should not have been allowed to pull out of hosting the Paralympic sport in 1980, but was talked around to taking part by a friend. At the Games, held at short notice in Stoke Mandeville, England, Harris was entered for the discus, shot put and javelin. Before taking part in the events, he was chosen to take the Paralympic Oath on behalf of all the athletes, an event he described as a “phenomenal honour”. Although Harris did not finish in the top three in either javelin or shot put, his main focus for a medal was the discus. Before his final attempt he was sitting in sixth place, but his Paralympic record breaking throw gave him the gold medal.
Harris states that his outlook to life changed after the games, becoming less self-centered and in 1986 he joined up with his training partner and good friend Chris Hallam, who was also an outstanding Paralympic athlete having won gold in the pool at Stoke Mandeville. The two decided to raise funds for, and awareness of, disability sport by taking part in a 400-mile wheelchair trip around Wales in just 11 days. This led to a further fund raising event in 1997 when the two men completed 600-mile in 37 days. The funds they raised allowed for the building of national centre for disability sport to be built at the University of Wales Institute, at the Cyncoed Campus site.
In 1988 Harris took part in his third Summer Paralympics, joining the Great Britain team when they travelled to Seoul. He entered two events, the category 5–6 pentathlon and the discus. In the discus he threw 34.98m, to take silver, 1.70m behind the eventual winner Egypt’s Mohamed Abdulla Mohamed. He finished fourth in the pentathlon, but was later awarded the bronze medal.