Muir became the youngest track and field athlete to represent Canada at the 2008 Summer Olympics in Beijing, where she competed for the women's 400 metres. She ran in the second heat against six other athletes, including Italy's Libania Grenot and Botswana's promising track star Amantle Montsho. She finished the race in third place by one hundredth of a second ahead of Cuba's Indira Terrero, with her personal best time of 51.55 seconds. Muir advanced into the next round of the competition, as she secured the final mandatory qualifying slot in the second heat. Muir, however, fell short in her bid for the final, as she placed seventh in the semifinal rounds, with her slowest possible time of 52.37 seconds.