Mouncey decided to transition while in Qatar to compete in a 2016 Summer Olympics qualifying tournament, and began hormone therapy in November 2015. She provided news of her decision to her mother while still in Qatar, prior to informing other friends and family, and publicly identified as a woman in May 2016.
Women's handball
's guidelines are for 12 months of hormone therapy before a trans woman can compete in women's competition, but Mouncey's testosterone levels were well below the required levels of 10 nmol/L by July 2016. Mouncey hoped to play in women's competition in October 2016, and to be selected in Australia women's national handball team. Her request to play for the ACT representative team in October 2016 was refused by the Australian Handball Federation, citing insurance concerns because she was still three weeks short of the IOC guideline of 12 months of hormone therapy. On 27 May 2018, Mouncey scored three goals for Melbourne Handball Club in their win over University of Queensland Handball Club for the 2018 Oceanian Open Club Championship. In April 2018, Mouncey began training with the Australia women's national handball team in anticipation of the International Handball Federation clearing her to play in the 2018 Asian Women's Handball Championship in Kumamoto, Japan. Mouncey scored four goals in her first international game with the Australia women's team, a 24–32 loss to Kazakhstan on 30 November 2018. Mouncey played all six games and scored 23 goals as the Australians finished fifth, qualifying for the 2019 World Women's Handball Championship. She was not selected to play at World Championships. Mouncey explained that this was due to some of the team being uncomfortable sharing the showers and change room with her, but this was denied by Handball Australia.
Although the 2018 AFL Women's season did not start until well beyond the 12 month IOC guideline, Mouncey's nomination for the 2017 AFL Women's draft was declined based on the Victorian Equal Opportunity Act allowing gender discrimination "if strength, stamina or physique is relevant." Mouncey's testosterone levels had remained far below the IOC limit, and the Australian Football League had indicated they would use IOC guidelines on transgender players. Mouncey previously played for the Ainslie Football Club in the 2017 AFL Canberra women's season. The AFL indicated that she could play for Ainslie in 2018, and could nominate for the 2018 AFL Women's draft. In response, the Ainslie Football Club coach and president both stated to the media that there had been no concerns about Mouncey's strength, stamina and physique in the 2017 AFL Canberra women's competition. The late decision to exclude Mouncey left the AFL open to criticism from the public and the media, and the AFL Players Association called for clarification from the AFL on the eligibility of transgender players. There was confusion about whether the AFL had authority to approve Mouncey to play in other Australian football leagues in 2018, or whether their approval was necessary, contrasting with both the earlier indication that this would be acceptable and Mouncey's participation in 2017. In December 2017, Mouncey decided to move to Melbourne to advance her football career and her public speaking business called Transform Coaching and Speaking. Fairfax Media listed the denial of Mouncey's nomination as one of year's top 10 sports stories in Canberra. In a statement on 13 February 2018, the AFL supported Mouncey's request to play state league Australian rules football, such as VFL Women's, and stated that the decision to deny her entry into the AFL Women's draft was only for 2017. AFL Victoria later confirmed, after she began training with Darebin Falcons, that Mouncey would play in the 2018 VFL Women's season. This was the first time that the AFL had agreed for an openly transgender woman to play at that level. Mouncey debuted for Darebin in round 1 of the 2018 VFLW season, kicking a goal while playing in the ruck and at full-forward in a lop-sided loss against the Northern Territory Football Club. Mouncey ended the season second in VFLW goal-kicking behind Darcy Vescio. On 9 September 2018, Mouncey announced that she would not nominate for the 2018 AFL Women's draft, while publicly releasing health and fitness records as evidence that she was eligible under the AFL's policies for nomination of trans women.
