At just 17 William Henzell was selected for his first senior team for the Oceania Olympic Qualifying Tournament in 1999. Although he missed out on a position on the 2000 Olympic Team, Henzell went on to play professionally in Sweden for 10 years. Henzell was selected for the Athens 2004 Olympic team and was the first Australian male to win matches in both the singles and doubles events, finishing 17th in the singles. Henzell won a silver medal at the 2006 Commonwealth Games and competed at the 2008 Beijing Olympic Games in the singles and team events, placing 33rd and ninth respectively.
Competing in his record third Olympic Games, Henzell had his best performance in men's singles in London. He cruised through the first and second rounds, taking out Hungary's Adam Pattantyus and Joao Monteiro of Portugal respectively. He then faced Belarussian Vladimir Samsonov in the third round, eventually going down in a thrilling seven set match 4-3. Henzell also contested the men's team event with Justin Han and Robert Frank. The trio were however defeated in the first round by the tough Singaporean side.