The former US secretary of state, 72, was speaking in London last night alongside her daughter Chelsea to launch the book they have co-written.
The Book Of Gutsy Women: Favourite Stories Of Courage And Resilience is about the women who have inspired them, including the 16-year-old Swedish founder of the international youth movement against climate change.
Clinton said: “I thought we’ll put her in the book because we had read about her lonely, solitary climate strike in front of the Swedish parliament and we were so moved by it.
“It’s been fascinating to watch how scared a lot of grown male leaders are of this young 16-year-old girl.”
Clinton became the first woman to represent a major US political party in a presidential race when she took on Donald Trump in 2016.
She said the fact Thunberg is a young woman speaking out is “rattling” the “ancient DNA” that still exists in society.
“It is maddening to think how much that still operates,” she said.
“You could probably take some of the people who have been so critical of her on social media and in other settings, attach them to a lie detector and say, ‘Don’t you think that’s a bit sexist?’
"And they’d say, ‘Of course not’, and they might even pass because they’re so enthralled to the idea that whatever she’s saying, before we even get to the merits of it, has to be discredited because she should not be saying it.”
Her daughter, 39, told the Royal Festival Hall on the South Bank she admired Thunberg’s “fearlessness” in campaigning.
“In our country there are a lot of people who are attacking her because they can’t attack the data, they can’t attack the science,” she added.
The event was hosted by historian Mary Beard. She also appears in the book, which features short essays about more than 100 women.
Among the other names included are sporting pioneers such as Billie Jean King and historical figures such as Florence Nightingale, as well as politicians, civil rights campaigners and suffragettes.
Asked which men she believes are “gutsy”, Clinton named former South African president Nelson Mandela and former US president Barack Obama.
“I think he was a very gutsy person but who worked so, so hard not to let it show,” she said. “He knew the undercurrents that were working against him and he’s been a great defender of women’s rights and opportunities.”