Mark Tewksbury was inspired to take up swimming after watching the sport performed at the 1976 Summer Olympics. He joined the national team in 1984 and, two years later, became a double gold medalist at the 1986 Commonwealth Games by winning the 100 metre backstroke and the 4x100 metre relay (alongside Alex Baumann, Victor Davis, Sandy Goss, Tom Ponting, Mike West, and the non-Olympians Claude Lamy and Darcy Wallingford). The following year he took the 100 m title at the Pan Pacific Championships, as well as silver in the 200 metre backstroke. His next stop was the 1988 Summer Olympics, where he was fifth in the 100 m, twelfth in the 200 m, and won a silver medal with Davis, Goss, and Ponting in the 4x100 metre medley relay. After earning bronze in the 100 m at the 1989 Pan Pacific Championships, Tewksbury defended his Commonwealth titles in 1990, with the assistance of Ponting, Marcel Gery, and Jon Cleveland in the 4x100 m relay. He won silver medals in the 100 m at 1991 World and Pan Pacific Championships, losing both times to Jeff Rouse of the United States, but won a silver medal in the 4x100 m relay with Cleveland, Gery, Ponting, and Stephen Clarke at the 1991 Pan Pacific Championships. At the 1992 Summer Olympics, Tewksbury defeated Rouse for the gold medal in the 100 m and captured bronze in the 4x100 m relay, alongside Clarke, Cleveland, Gery, and Ponting.
Tewksbury retired from active competition after the Games, but his saga with the Olympics was far from over. He worked with the IOC for several years as an athlete representative, but turned into a vocal critic of the organization in 1999, vowing to fight against its corruption. After coming out as homosexual in 1998, a move that cost him a six-figure speaking deal, he became an ardent supporter of gay rights, particularly in athletics, and was co-president of the first World Outgames in 2006. His many honors include appearing on the cover of TIME Magazine, an honorary law degree from the University of Western Ontario, receiving the Lou Marsh Trophy in 1992 as Canada’s top athlete, selection as the Chef de Mission for the 2012 Summer Olympics, and induction into the Canadian Olympic, Canada’s Sports, the International Swimming, Canada’s Q, and the LGBT Human Rights Halls of Fame.