Milla Jansen won gold in 50 meter freestyle at the 2022 Junior Pan Pacific Swimming Championships.[1]
On 25 October 2024, she beat the Australian record for her age range on 200 meter short course free style.[2]
She was part of Australia's swimming team that took silver in the 4×100 meter freestyle and bronze in the 4x200 meter freestyle at the 2024 World Aquatics Swimming Championships in Budapest.[1][3]
Jansen made her senior Long Course team debut at the 2025 World Aquatics Championships in Singapore. She won a gold medal swimming the final of the women's 4x100 meter freestyle relay, and was also awarded a gold medal for swimming the heats of the 4x200 meter freestyle relay.
