Now, just under twelve months on from that moment, she is.
After taking a year-long break from education to try and force world leaders to take action on climate change, Greta has returned to class.
She posted a picture on Instagram with the caption "My gap year from school is over, and it feels so great to finally be back in school again!"
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Greta's been busy in the last twelve months by anyone's standards, but what exactly has she been up to and how much impact has her work had?
August 2019: Sailing to New York
Media captionGreta Thunberg sails into New York City
Plenty of students on a gap year enjoy a nice boat trip, but not quite like this one.
Her break from the classroom started in August 2019 with a 3,000-mile (4,800km) voyage across the Atlantic.
Then aged 16, she sailed from Plymouth to New York on a zero-emissions yacht, to speak at UN climate summits in New York City and Chile. and she wanted to make sure the carbon footprint of her travel was as small as possible.
"Our war on nature must end," she said after arriving in the Big Apple.
Early September 2019: That UN speech
Media captionWATCH: Greta gives a passionate speech to world leaders
While in the US, Greta spoke at the United Nations Climate Action Summit.
It was a speech that demanded action, and she told the audience: "I shouldn't be up here."
"I should be back in school on the other side of the ocean,
"Yet you all come to us young people for hope. How dare you. You have stolen my dreams and my childhood with your empty words."
Greta also bumped into US President Donald Trump, but didn't appear to be his biggest fan.
Late September 2019: Global climate strike
Media captionMontrealers take to the streets in protest
From the USA, to Canada - where Greta led a rally as part of the Global Climate Strike in Montreal.
Hundreds of thousands of people joined marches across Canada, and almost a hundred other events took place in cities and towns across the world.
It all started with school strikes, inspired by Greta's "Fridays for Future" movement.
Officials in Montreal said around 315,000 people gathered, making it one of the most attended environmental marches in history.
She also called out her social media critics.
"The haters are as active as ever", she said.
"Going after me, my looks, my clothes, my behaviour and my differences". Anything, she says, rather than talk about the climate crisis."
December 2019: A second boat trip
Media captionGreta Thunberg: Climate activist speaks as she arrives in Europe for COP25
Greta had stayed in the Americas to speak at the United Nations Climate Change Conference, which had been due to take place in Chile.
Because of big civil unrest there it was moved to Spain, but Greta wasn't about to grab her boarding pass and hop on the next flight.
Instead, after making a social media appeal for a lift across the Atlantic she was offered a ride by a family of YouTubers who post videos about their journey around the world.
"We need to work together to make sure that we secure future living conditions for humankind, and that we fight for not only ourselves, but for our children and for our grandchildren and for every single living being on Earth," she told the crowd after arriving.
2020: Taking on Trump and the EU
At the start of 2020, Greta guest-edited the UK's second most listened to breakfast show, BBC Radio 4's Today Programme, which the BBC turned in to a special podcast,..
In January she joined the most powerful economic leaders on the planet at the World Economic Forum in Switzerland.
It was there she once again caught the attention of President Trump. Both spoke at the convention, but their message couldn't be further apart.
In March it was another big meeting, the European Parliament's Environment Committee, where she criticised the EU's plan for tackling climate change, telling Members of European Parliament it amounts to "surrender".
And not even a global pandemic could stop Greta's protests. In March she announced plans for a "Digital Strike".
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'The Greta effect'
No changes to any government's policies on climate change have been directly attributed to Greta or her activism, but some believe her work in bringing the topic to public attention has been significant.
At the end of 2019, science magazine The New Scientist said that 2019 was the year the public "finally woke up to climate change," largely thanks to the work of Greta and the Extinction Rebellion protest group.
Major politicians, like UK Environment secretary Michael Gove and former Labour leader Ed Miliband have praised the impact of her work, often known as "The Greta Effect."
And perhaps most significantly, Greta shared a stage with European Commission President Jean-Claude Juncker, where he promised billions would be spent on tackling climate change.
While it's back to school for now, don't bet on Greta letting the world's politicians off the hook. After all, as she's promised in the past, "we'll be watching you."