For more than three decades, she has worked to save Africa's natural environment and improve the quality of life for women.
But the Nobel Committee's announcement today caught her completely by surprise.
"I was shaking and crying," she told ABC News. "I am still very excited and I particularly like the fact the news reached me here in Nyeri, at home, in front of Mount Kenya."
She is known as "the Tree Woman of Kenya," sometimes as "Mother Earth."
Her Green Belt Movement has planted 30 million trees across Africa to combat deforestation, which has been devastating to the African way of life.
Half of Africa's forests have been destroyed. Much of the continent has been devastated by mining, logging and other development and people in need of fuel.
Maathai believes that saving the environment is a way to safeguard peace.
"When we destroy our resources, when our resources become scarce, we fight over them. And many wars in the world are actually fought over natural resources," she said.
Maathai began planting trees in 1977 — in her own back yard at the foot of Mount Kenya.
By the early 1980s she had encouraged the formation of 600 nurseries growing trees.
Some 3,000 women make a little money caring for the trees. Maathai raises all the money privately. By 1993 more than 20 million trees were growing.
"She is like the Pied Piper. Whenever she visits a village, the women gather around her, they sing to her when she arrives, they give her great tributes, food, and just love," said friend Mary Davidson.
Maathai is the first African woman to win the Nobel Peace Prize.
She is the first Kenyan woman to get a Ph.D. She is the first woman to become a professor at the University of Nairobi. She studied biology, she said, because "I have a mind that tends to ask why."
She has won many environmental awards, including the very important American Goldman Prize.
"She has had such a meaningful effect on the people of Africa and beyond," said philanthropist Richard Goldman. "She is a marvelous, totally dedicated human being, and she will leave a real legacy for what she has been doing."
Maathai said: "We have so many problems but we are struggling, we are trying and we are addressing a very important aspect that is the environment."
It has not always been easy. Former Kenyan President Daniel Arap Moi called her a threat to the order and security of the country.
And when she takes on the government and developers she has been clubbed and tear-gassed and arrested.
Her parents were farmers. She says she had a very average childhood.
In 1960 she was chosen as one of 300 Kenyans to study in America.
She and her husband are divorced. "She was too educated, too strong, too successful, too stubborn, and too hard to control," he said. She was denounced by the Kenyan government for refusing to submit to men.
Yes, she's a tough one. "I know that people out there believe in me, they believe in the values I have been representing and they want to see me make a difference," Maathai said.
ABC News' Peter Jennings filed this report for World News Tonight.