Samantha Joy Mostyn (/ˈmɒstən/ MOSS-tən; born 13 September 1965) is an Australian businesswoman and advocate, who has been serving as the 28th governor-general of Australia since 1 July 2024.

Mostyn has been an 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. The Mostyn Medal, for the "best and fairest" AFLW player in the Sydney Swans, is named after her.

Samantha Joy Mostyn was born on 13 September 1965 in Canberra, the eldest of four sisters. One of her sisters has an intellectual disability, so the family was involved in the disability sector. Their father, William "Bill" Mostyn, was a graduate of the Royal Military College, Duntroon, and a colonel who served for almost 40 years in the Australian Army. As a major in the Royal Australian Signal Corps, he served in Headquarters Australian Force Vietnam during the Vietnam War.

Although most of her early years were spent in Canberra, being the family of a military man meant moving frequently, and included two years of Mostyn living in Adelaide with her grandmother while her father was in Vietnam. She also lived in Melbourne, the United States, and Canada. She played a lot of sport as a child, and loved to watch Australian rules football, although she did not have the opportunity to play. She has recalled attending the ANZAC Dawn Service each year with her family. Mostyn attended Narrabundah College. While earning her arts and law degree at the Australian National University, she worked as a researcher for local chief magistrate Ron Cahill.

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. Her work has included roles in business strategy, human resources, culture change, risk management, and community engagement.

After leaving university, Mostyn trained as a solicitor while working part-time in the Magistrates Court of New South Wales and later as an associate to Michael Kirby in the New South Wales Court of Appeal. She worked as a solicitor for Freehills and Gilbert + Tobin.

In 1992, Mostyn joined the office of transport and communications minister Bob Collins as a senior policy adviser, specialising in intellectual property and also advising on the introduction of pay television to Australia. She subsequently moved to the office of communications and arts minister Michael Lee, before briefly joining the Seven Network as a broadcast policy manager. In 1995, Mostyn was recruited by prime minister Paul Keating to work in his office as a communications policy adviser. She was also appointed by Keating to the board of the organising committee for the 2000 Summer Olympics in Sydney, where she served until 1996.

In 2022, Mostyn was appointed by the Albanese government as chair of its Women's Economic Equality Taskforce. In 2023, this taskforce recommended that paid parental leave should be extended to a year.