Shawn O’Sullivan was crowned the Canadian junior champion in the light-middleweight division by the age of 16. By the early 1980s he was representing Canada abroad and was crowned the World Amateur Champion, as well as Canadian Athlete of the Year, in 1981. He won gold at the 1982 Commonwealth Games and was considered one of the favorites, along with American Frank Tate, in the division at the 1984 Summer Olympics. The Canadian fought Tate in a battle that most spectators, including Tate’s coach, believed that O’Sullivan won.
O’Sullivan turned professional after the Olympic Games and won sixteen of his first seventeen fights, losing only to Simon Brown, a future WBC Welter and Light-Middleweight Champion. He retired in 1997 with a career pro record of 23 (KO16)-5.