The businesswoman, lawyer and former AFL commissioner was announced as David Hurley's replacement in April and today formally began the role during a swearing-in ceremony at Parliament House.
In an uplifting first speech as governor-general, Mostyn called on Australians to take a caring, collective approach to the difficult conditions many households are facing at the moment.
"These testing times call for an unstinting focus on kindness, on care and on respect," she said.
"Across my career and particularly in the past decade, I have seen how care can be an uplifting force. I have seen it in the reform of workplaces, where inclusion and respect now prevail.
"I have seen it in our renewed focus on the roles of teachers, nurses, care workers, volunteers and all frontline workers, not just during the crises of bushfires, floods and COVID-19, although very much amplified by those times.
"I have seen it in the advancement of women in all parts of society, in leadership roles, sport, in economics, our regulators, even slowly but importantly in the trades and on building sites."
She also spoke about the common ambitions Australians have for the country.
"Women and men and often children stop me regularly to talk about this country they love. Across a wide range of backgrounds, life experiences and opinions, all of them wished for a brighter future for Australia," she said in her address.
"A desire for unity and optimism, a renewed sense of national possibility in building the future together.
"It struck me that the former governors-general whom I met expressed the same desires for this country in almost the same words as did everyday Australians.
"They also reminded me that the role of governor-general is not simply to be an observer of Australian life, but to be a participant.
"To reflect the Australian character and its fundamentally democratic spirit."
Prime Minister Anthony Albanese said Mostyn was the "right leader" for Australia, describing her as a "person of intelligence and compassion, of loyalty and integrity".
"Throughout your life and across your great breadth of experience in the worlds of law and business and sport, the not-for-profit sector, you have always been ready to put yourself forward for others," he said.
Mostyn is the second woman to become governor-general, following in the footsteps of Quentin Bryce, who held the role between 2008 and 2014.
A lawyer by trade, she has worked across a wide range of companies in both the private and public sectors, including at Optus, the Climate Council, IAG, Mirvac and the AFL, where she was the first woman to be named to the sporting code's commission in 2005.
She has been an advocate for action on climate change, reconciliation and gender equality, and has also expressed support for Australia becoming a republic.
