Elizabeth Dowdeswell arrived at the legislature in a horse-drawn landau for her installation ceremony, which included a 100-person honour guard from the 4th Canadian Division and a 15-gun salute.
She was greeted by outgoing Lt.-Gov. David Onley, who had held the vice-regal position for the past seven years.
In the text of her inaugural speech, Dowdeswell applauded all her predecessors for fighting racism and advocating on behalf of First Nations, children and the environment.
Dowdeswell said she will break with the tradition of incoming lieutenant-governors declaring during their investiture the themes that will be their priorities.
Instead, she will start her term by convening diverse groups of Ontarians to hear ideas about the province's place in the world.
"What can we contribute and what we can learn from each other to meet the global and local challenges we face in common?" she said. "The aim will be to provide a forum for reflection in the lieutenant-governor’s office, a safe space and a crucible for ideas. And in time priority themes will emerge."
Dowdeswell said her time as undersecretary general of the United Nations helped shape her world view, and convinced her "the world's most important problems are Ontario's most fundamental challenges."
"Our first challenge is to ensure responsible and inclusive prosperity so that everyone has a meaningful opportunity to participate," said Dowdeswell.
"The second and related challenge is the fragility of our planet, our mutual vulnerability. Finally, there is a third challenge: as we protect our precious natural resources and develop economically, can we ensure social cohesion?"
Premier Kathleen Wynne said she is pleased the new lieutenant-governor is someone "who is so passionate about public service."
"She has championed a thoughtful approach to improving people's lives through public policies to protect the environment, improve the quality of education and promote sustainable energy development," Wynne said in a statement.
Dowdeswell has a broad range of experience in Canada and internationally, including positions as deputy minister of culture and youth in Saskatchewan, former assistant deputy minister of Environment Canada and the first woman to head the UN Environment Program.
Born in Northern Ireland, the 69-year-old moved to rural Saskatchewan with her large family at a child. Dowdeswell started as a home economics teacher and moved up to deputy minister of culture and youth. Dowdeswell was president and CEO of the Council of Canadian Academies when her appointment was announced. Prior to that she was the founding president and CEO of the Nuclear Waste Management Organization.
The Progressive Conservatives said the "diversity of her accomplishments" sets the stage for distinguished service.
"We eagerly anticipate the positive initiatives transcending politics that will emanate from her vision," Interim leader Jim Wilson said in a statement.
NDP Leader Andrea Horwath said Dowdeswell will be a "great role model" for young girls in Ontario.
Note to readers: This is a corrected story. An earlier version said Dowdeswell was born in Saskatchewan.