2015-2016 Mizzen won the gold medal on the uneven bars at the 2015 Australian Championships. She then made her senior international debut at a friendly meet against China and placed seventh on the uneven bars. Mizzen was selected to represent Australia at the
2016 Olympic Test Event alongside
Georgia-Rose Brown,
Emily Little,
Larrissa Miller,
Kiara Munteanu, and
Emily Whitehead to try and qualify Australia a team spot for the
2016 Olympic Games. The same team competed at the
Pacific Rim Championships to prepare for the Test Event, and they won the bronze medal behind the United States and Canada. At the Test Event, the team finished in fifth place and
did not qualify a full Olympic team. At the Australian Championships, she won the gold medal in the all-around and on the uneven bars. However, she was not selected for Australia's sole Olympic spot. She won the gold medal on the uneven bars at the
Anadia World Challenge Cup.
2017 Mizzen won the bronze medal on the uneven bars at the
Melbourne World Cup behind Chinese gymnasts
Liu Tingting and
Luo Huan. She then went to
Montreal for the International Gymnix and placed ninth in the all-around and sixth on the
floor exercise. She also won the bronze medal on the uneven bars behind
Hitomi Hatakeda and
Jade Chrobok. Then at the
Baku World Cup, she won the silver medal on the uneven bars behind
Diana Varinska. She then finished fifth on the uneven bars at the
Doha World Cup. At the Australian Championships, she won the bronze medal in the all-around behind Emily Little and Georgia-Rose Brown. She also won the gold medal on the uneven bars, placed seventh on the
balance beam and won the bronze medal on the floor exercise. She was selected to compete at the
World Championships, but she did not qualify for any finals.
2018 Mizzen competed at the
Melbourne World Cup and finished eighth on the uneven bars. She was selected to represent
Australia at the 2018 Commonwealth Games alongside Georgia-Rose Brown,
Alexandra Eade,
Georgia Godwin, and Emily Whitehead. Mizzen and Godwin were both sick at the training camp immediately prior to the Games, but they recovered and were still able to compete. The team won the bronze medal behind England and Canada. At the Australian Championships, she won the bronze medal in the all-around behind Georgia Godwin and Emily Whitehead, and she won the silver medal on the uneven bars behind Godwin. Then at the Australian Classic, she won the silver medal in the all-around behind Georgia-Rose Brown and the gold medal on the uneven bars. She was initially selected to compete at the
World Championships, but she
tore her ACL and withdrew. == Retirement ==