At first, she began protesting outside the Swedish Parliament, inspiring a movement called Fridays for Future, which thousands of children have joined to demand action on climate change.
That was a year ago. On Wednesday, Greta Thunberg completed an ocean odyssey by arriving in New York, where she will take up an invitation to speak at the UN.
Soon after Greta, 16, shakily disembarked and stepped dockside in Manhattan, she remarked how she is always asked about US President Donald Trump and what she would like to tell him.
"My message for him is listen to the science and he obviously doesn't do that," she said, soon after a cheering crowd had chanted her name under overcast skies in the final moments of a 5556-kilometre trip.
"It's insane that a 16-year-old has to cross the Atlantic Ocean to make a stand. This, of course, is not something that I want everyone to do."
She had passed the Statue of Liberty and headed up the Hudson River before docking at North Cove Marina, not far from Wall Street, shortly after 4pm.
The UN sent a flotilla of 17 sailboats, one for each of its sustainable development goals for 2030, to welcome her.
The young activist made land after a 15-day journey on Malizia 11, a racing yacht equipped with solar panels and underwater turbines that do not produce carbon. Its black sails carry the message: Unite Behind the Science.
She also made time for a few jokes at the US president's expense, remarking that she was “pretty sure” that wind turbines do not cause cancer, debunking a claim that climate change sceptic Mr Trump made at a Republican rally in April.
Greta, who had Asperger's syndrome diagnosed at the age of 12, began sitting outside the Swedish Parliament in August 2018, asking representatives to act on climate change.
She will take part in a UN summit on zero emissions on September 23, but refused to fly to New York because of the carbon footprint left by planes.
Instead, she was offered a ride on the Malizia II, skippered by Pierre Casiraghi, the son of Princess Caroline of Monaco, and German round-the-world sailor Boris Herrmann.
"Land. The lights of Long Island and New York City ahead," Greta tweeted early on Wednesday alongside a dark, blurry photo of lights in the distance.
She later tweeted that her yacht had anchored off Coney Island in Brooklyn to clear Customs and immigration.
The teenage activist arrived about 2.45pm local time after the voyage.
She is due to travel to Chile for another UN climate summit later this year, but Greta's team are wondering how easy it will be to make the journey there and back to Sweden on zero carbon emissions.
Greta's uncompromising attitude has won her friends and foes alike. Critics took aim at her when single-use plastic bottles were seen on the yacht.
Others said that encouraging children to skip school and take part in co-ordinated protests was not an appropriate way to make a stand.
But that is what is scheduled to happen on Friday outside UN headquarters in Manhattan. After a day's rest on Thursday, Greta said she would be there.