On 10 August 1965, aged twelve years, Muir became the youngest person to break a sporting world record in any discipline when she swam the 110 yards backstroke in 1m 08.7s at the ASA National Junior Championships in Blackpool, England.
Over the following five years Muir would go on to set fifteen world records in the backstroke at 100 metres, 200 metres, 110 yards, and 220 yards. She also won 22 South African Championships and three US National Championships. Due to the sporting boycott of South Africa during her active career, she was never able to participate in an Olympic Games.
Muir was elected to the International Swimming Hall of Fame in 1980. After retiring from her sport, she qualified, through the University of the Orange Free State, as a doctor and practiced in the African continent. Since 2000 she worked as a family physician in Vanderhoof, British Columbia, Canada. During 2009, she was diagnosed with breast cancer. On 19 January 2012, it was reported that the cancer had spread.
Muir died of breast cancer at the age of 60 in Mossel Bay, South Africa on 1 April 2013.
Kimberley's Olympic-sized swimming pool was named the Karen Muir Swimming Pool in honour of the young swimmer, who was nicknamed locally as the "Tepid Torpedo". When Muir revisited the city in 2009 she donated her Springbok blazer to the Diamantveld High School.
- List of members of the International Swimming Hall of Fame
- World record progression 100 metres backstroke
- World record progression 200 metres backstroke
- Media related to Karen Muir at Wikimedia Commons