Coming from Kingaroy, Queensland, Thorn spent the majority of her career in the shadow of fellow Australian Dawn Fraser. Making her debut in the 1962 Commonwealth Games in Perth, Thorn was part of the winning 4x110yd freestyle relay team, and won silver in the corresponding individual event behind Fraser. Two years later in Tokyo, Thorn was a semifinalist in the 100m freestyle, won by Fraser. She combined with Fraser, Lyn Bell and Janice Murphy to finish second, 3.1s behind the United States.