Mostyn has been an outspoken advocate on climate change and gender equality; she served as the first female Australian Football League commissioner and was president of Chief Executive Women from 2021 to 2022.

She was a board member of numerous companies and organisations, including Mirvac, Transurban, GO Foundation, the Climate Council, Virgin Australia, and the Sydney Swans.

[8][9] Their father, William "Bill" Mostyn,[10] was a graduate of the Royal Military College, Duntroon, and a colonel who served for almost 40 years in the Australian Army.

[16] While earning her arts and law degree at the Australian National University, she worked as a researcher for local chief magistrate Ron Cahill.

[3][17] Mostyn has held many non-executive roles in business and government, and has also been involved with advocacy organisations and issues that relate to climate change, gender equality, Indigenous reconciliation, and environmental sustainability.

She subsequently moved to the office of communications and arts minister Michael Lee, before briefly joining the Seven Network as a broadcast policy manager.

[20] As the company's director of government and corporate affairs, she was named "one of the most powerful women in the information technology industry" in 1998 by the Australian Financial Review.

She served as a commissioner until 2016 and was a key figure in the development of the Australian Football League's Respect and Responsibility Policy, as well as an advocate for the creation of the AFL Women's competition.

[a][19] In 2021, Mostyn was named by the Australian Financial Review as Australia's "most influential" company director, serving on boards with a combined market capitalisation of over $480 billion.

[19] On 3 April 2024, Prime Minister Anthony Albanese announced that King Charles III had approved the appointment of Mostyn as the next governor-general of Australia, succeeding David Hurley, and that she would be sworn in on 1 July 2024.

The announcement was generally welcomed by other politicians,[43] including by the leader of the federal Opposition, Peter Dutton;[4] Mostyn's colleagues; several women's advocacy organisations;[44][43] the AFL;[26] the Vietnam Veterans Association of Australia;[10] the president of the Law Society of New South Wales;[45] the incumbent governor-general, David Hurley,[46] and others.

Mostyn commented that recommendations by Kate Jenkins, Sex Discrimination Commissioner, following the national inquiry into workplace sexual harassment, could be implemented and accepted.

[58] Mostyn delivered a speech at the National Press Club, in November 2021,[59] as president of Chief Executive Women, on economic recovery and post-pandemic recovery, describing how Australia can make "the most of its available resources and talent" by investing in care, for paid parental leave, childhood education and superannuation reform, as well as ensuring employees in the care industry, such as teachers, childcare workers and nurses, are receiving well-paid salaries, and respect within the workplace.

[64] She commented that the election in 2022 would be a gendered issue, signing an open letter saying that widespread reform is needed to assist the return to the workplace for Australian women.