Fairweather began competing in triathlons in 1992 and won the elite Australian National Series in her first season. She spent eight years as a professional triathlete. In 1996 she became the world triathlon champion, setting a championship record time of 1 hour 50 minutes 52 seconds in Cleveland, Ohio, United States. She also won the World Duathlon Championships in 1996 to become the only person ever to win both world titles in the same year. Fairweather won the Duathlon World Championships again in 1999, but missed repeating the double when she placed 2nd to Loretta Harrop in the Triathlon World Championship. Gallagher collected further World Championship silver medals in 1995 and 1997.
In 2001, Fairweather was appointed to the position of Head Coach of the newly established Australian Institute of Sport triathlon program. From 2005 to her death on 2014, she held senior sport administration positions with the Australian Sports Commission /Australian Institute of Sport. These positions primarily assisted the high performance programs of national sports organisations. Fairweather played a major role in triathlon administration and positions included: Triathlon Australia national elite selector, International Triathlon Union Athletes Committee athlete member, ITU Women's Committee member and Triathlon Australia Board Member .
Death
On 4 November 2014, it was announced that Fairweather had committed suicide at the age of 46 on 1 November 2014, nine days before her 47th birthday. Her death led to many former athletes recognizing her achievements. Her Australian triathlon rival in the 1990s, Emma Carney said "Jackie was a perfectionist and I think perhaps Jackie never really appreciated or realised all that she achieved. It wasn't just the races she won, or the sports she excelled in Jackie was a pioneer in Australian triathlon coaching and administration." Emma Snowsill, 2008 Beijing Olympics Women's Triathlon gold medalist commented that "You shared and cared in your knowledge and passion for our sport more than anyone Jackie Fairweather. Not only a hero for your athletic abilities but your generosity to help many up and coming athletes to pave a way for themselves and the future of this sport is second to none." Snowsill was a member of Fairweather's Australian Institute of Sport triathlon squad. Her Memorial Service was held at the AIS Arena with more than 600 people attending. The location was apt as it was where Jackie worked for 13 years and met her husband Simon.