Mary Beth Pauline Stewart (born December 8, 1945), also known by her married name Mary McIlwaine, is a Canadian former competitive swimmer who twice broke the world record in the women's 100-metre butterfly in the early 1960s. Stewart also competed in freestyle events as a member of the Canadian national team in major international championships.
As a 14-yard-old, she represented Canada in the 1960 Summer Olympics in Rome, where she finished eighth in the women's 100-metre freestyle, and also competed in the preliminary hears of the women's 4x100-metre medley relay event. Two years later at the 1962 Commonwealth Games in Perth, Australia, she won the gold medal in the women's 100-metre butterfly. At the 1963 Pan American Games in São Paulo, Brazil, Stewart enjoyed a four-medal performance, garnering silver medals for her second-place performances in the 100-metre freestyle, 100-metre butterfly, 4x100-metre freestyle relay, and the 4x100-metre medley relay.
In her final international appearance at the 1964 Summer Olympics in Tokyo, she finished eighth in the 100-metre butterfly, and also competed as a member of the Canadian teams in the 4x100-metre freestyle and 4x100-metre medley relay events.
She is the sister of Helen Stewart.