At the outset of his very, very long walk, during which he decided to stop speaking for 17 years, some people began to suspect John Francis had lost his mind. Others lauded him for embarking on an enlightened journey to save the planet.

In hindsight, the optimists proved correct. Since embarking on his quest, he’s served as a United Nations goodwill ambassador for the environment and a National Geographic explorer. He’s given a Ted Talk, commencement speeches and earned a Ph.D. 

He’s known as the Planetwalker, and people look to him for inspiration. Carrying his beloved banjo, dubbed “The American Princess,” he’s traversed the entire United States on foot and walked across Cuba and the length of South America. Now, at age 77, he’s walking in stages from Cape Town to Cairo.

Next week, Dr. Francis is returning to Inverness, where his remarkable journey began. He will give a talk at St. Columba’s Episcopal Church and lead a weekend retreat hosted by Planetwalk, the nonprofit he started to inspire environmental activism and education. 

His odyssey started in West Marin in 1971, after two Standard Oil tankers collided, spilling more than 800,000 gallons of fuel into the San Francisco Bay and coating thousands of shorebirds with thick black paste. 

He saw people on the shore of Bolinas Lagoon piling up dead birds and trying in vain to rescue those that tried to flap their oil-laden wings.

Not long after witnessing this spectacle, Dr. Francis, who was 27 at the time, vowed to stop driving or riding in cars. He’d been living in Inverness with some hippie friends, smoking weed and managing a jazz band. The horrific scenes he witnessed on that grim winter day galvanized him with a new purpose. 

His sense of urgency was reinforced when a neighbor drowned in Tomales Bay. “There’s no guarantee that you’re going to have tomorrow,” he said. “You only have right now.”

Speaking with the Light by phone this week, Dr. Francis shared the fundamental lesson he has learned during his journeys: The key to saving the planet is kindness. We’re all part of the environment, he said, and to save the earth we must start by treating one another with decency. 

“Let’s take care of each other,” Dr. Francis said from his home in New Jersey, which was shrouded with smoke from Canadian wildfires. “If we can be kinder to each other, then we can solve a lot of the issues that are plaguing our planet.”

He sees a nexus between environmentalism and issues such as human rights, economic equity and gender equality. 

“If we’re oppressing one another and exploiting each other, then that’s going to manifest in the environment,” he said.

In the beginning, his walks were limited to West Marin. One night, he and a friend—who was an oil heiress—walked to San Anselmo and back to go dancing. If he needed something in town, he would walk the four miles to Point Reyes Station. He walked to Petaluma and back for a doctor’s appointment. More than once, he walked up to Oregon and back.

People often stopped to offer him a lift. Some got angry when he turned them down, thinking he wanted to guilt them for burning gasoline. Friendly or mean, they all received a silent “no thanks” from Dr. Francis, who would communicate with gestures.

Cindy Ohama, who lived in Inverness at the time, used to see him walking around town. She was frightened at first by his peculiar gesticulations. But it turned out that Dr. Francis was friends with the man Ms. Ohama and her three young children were living with, and he came to their house for dinner.

“I learned to interpret his pantomime,” Ms. Ohama recalled. “He was really good at it. When I didn’t understand, I’d make him write it down. We got to be good friends.”

Eventually, Dr. Francis asked Ms. Ohama, who was working as a financial consultant to small businesses, to help establish his nonprofit.

A lawyer in San Francisco helped them with the paperwork. Ms. Ohama drove from Inverness to the attorney’s office. Dr. Francis walked. Somehow, they made their way through the paperwork.

Despite his unconventional behavior, Ms. Ohama didn’t think he was crazy. “This was the ’70s,” she said. “Times were different then. There were a lot of people doing unusual things. His was one of the most unusual, but it wasn’t like completely out of line with what you’d find in West Marin.”

Dr. Francis, who is African American, had a few frightening encounters during his travels, including the time two white men in Oregon held a gun to his head and spewed racial slurs. But mostly, he was greeted by generous people who offered him food, shelter and encouragement.

“If people were not kind to me,” he said, “I don’t think I would be here.”

Somehow, even while forswearing speech, he earned his bachelor’s degree from the University of Oregon and later received a Ph.D. from the University of Wisconsin. As the media chronicled his journey, he became an environmental celebrity. 

As he would later explain in his Ted Talk, Dr. Francis had only intended to remain silent for one day. “It was a very moving experience, because for the first time, I began listening,” he said.

He remained silent for another day, and then another. “I promised myself for a year I would keep quiet, because I started learning more and more, and I needed to learn more,” he said.

He renewed his vow of silence each year. Finally, in 1990, on the 20th anniversary of Earth Day, he broke his silence, much to the joy of his parents. Not long afterwards, he was hired to help write oil spill regulations for the U.S. Coast Guard, a job he kept for a year before embarking on more transnational walks. 

In all, Dr. Francis has spent 24 years in West Marin, including five years with his wife, Martha, and two sons, who spent their early years living in Point Reyes Station before moving to New Jersey in 2010.

These days, Dr. Francis is frequently called upon to make public appearances. He’s the subject of an upcoming documentary called “John Francis and the American Princess.” The directors plan to submit the film to the Sundance Film Festival later this year.

Last week, Dr. Francis was interviewed on a podcast hosted by GirlTrek, a national nonprofit that promotes Black women’s health through walking. Its “Black History Bootcamp” podcast has won two Webby awards. The theme of his episode was stillness.

“Stillness for us is really about being able to connect with nature, being able to be quiet at times to just gain healing from time spent in the great outdoors,” said Marcie Thomas, the organization’s director of digital engagement. “He’s the Planetwalker. He’s a pioneer of Black conservation and environmentalism.”

The oil spill that propelled him into environmental movement inspired many people to engage in political action, Dr. Francis said. He hopes climate change will have a similar impact on the public today. 

“People who have seen these fires, who have seen destruction in the rain forest, might be propelled to go deeper into their spirit about what the environment is, and how to change its course,” he said.