“I was sitting at the Yak and Yeti bar of the Royal Hotel, and the prince mistook me for someone else.”

And so began the remarkable Himalayan journey of Barbara Adams.

The year was 1961. The onetime Washington schoolgirl was, at 29, a free spirit and an expatriate, having spent most of the previous decade in Rome.

When she caught the prince’s eye, she was in Kathmandu, on assignment for an Italian magazine to cover the state visit of Britain’s Queen Elizabeth II to Nepal. Ms. Adams stayed as the “royal consort” — or openly acknowledged mistress — of Prince Basundhara, the brother of the king.

She was a striking figure, then and always, with her blond hair, piercing blue eyes and charismatic nature. In Kathmandu, where bicycles and horses were the typical means of transport, Ms. Adams drove a white Sunbeam convertible.

Barbara Adams in Nepal. (Thomas L. Kelly)

Over the next five decades, she would become the best-known American in Nepal. She was sought out by diplomats, and for years, she held court at the boozy expatriate gatherings at her expansive house, which held two grand pianos, in downtown Kathmandu.

Ms. Adams founded Nepal’s first travel agency, wrote newspaper columns and established a foundation that built homes for impoverished, lower-caste citizens. She called for political change and, during one period of turmoil, was expelled from the country.

But she always found her way back, and she was in a Kathmandu hospital when she died on April 22, two days before her 85th birthday. She had a heart attack last year and died of complications from abdominal surgery, said her executor, Mike Gill.

Barbara Adams was born April 24, 1931, in New York City. She grew up mostly in Washington, where her father worked as an economist in the administration of President Franklin D. Roosevelt.

Headstrong and outspoken from an early age, Ms. Adams dropped out of high school and spent her days riding horses. She managed to get admitted to George Washington University and later studied languages at Columbia University, but she left the United States in the early 1950s to pursue a bohemian life in Europe.

She spent several years in Rome, where she wrote for both Italian- and English-language publications. She sometimes hinted that she was married to an Italian doctor, but that cannot be verified.

When Ms. Adams arrived in Nepal, it was still a desperately poor feudal society, known to the West mostly as the home of Mount Everest and other spectacular peaks of the Himalayas.

Barbara Adams in Nepal. (Thomas L. Kelly)

“It was a very magical and attractive place,” said Washington writer Kai Bird, who lived in Nepal for several years. “You could only get in by prop plane from India. It was very exotic and lush.”

When Ms. Adams met the urbane Prince Basundhara at the Yak and Yeti, he apologized for thinking she was someone else. Then he showed her pictures of a mountain-climbing expedition, and before long, they were inseparable.

It didn’t matter to Ms. Adams that the prince was married and had several children, or that he had a reputation as a playboy. Before long, they moved into a palace together, and she became known as the royal consort.

“It was the easiest thing in the world to turn my back on my old life,” she told Agence France-Presse in 2006, “because I never really cared for it.”

She and the prince set up the travel agency and explored the remote regions of Nepal together. When she lost touch with her family, a cousin who worked for the State Department was sent to find her. They met near the border of Nepal and India.

“Basundhara and I were on elephants,” she recalled in 2006, “and suddenly a Jeep came along, the first vehicle we had seen in three days, and it was my cousin Wesley Adams. It was a real ‘Dr. Livingstone, I presume’ moment.”

Ms. Adams never relinquished her U.S. citizenship and often returned to the United States, but Nepal became her spiritual home. She became fluent in the language and stayed after Prince Basundhara died in 1977.

She traveled often to the nearby country of Bhutan, collecting art and fabrics, which she restored at her home in Kathmandu. She eventually wrote a book on Bhutanese textiles and sold her collection to a museum.

In the 1980s, after Westerners discovered Kathmandu as a feel-good city with easy access to hashish and other mood-altering substances, the Nepali government sought to reduce the foreign presence in the country. Ms. Adams fell out of favor with members of the royal family and was told to leave.

Gill, a former Peace Corps volunteer who stayed on in Nepal, recalled “a strange woman” knocking on his door in the late 1980s. It turned out to be Ms. Adams, seeking refuge.

“She was dressed like an aging cheerleader, in saddle shoes, pleated skirt and a beehive hairdo, which turned out to be a wig,” Gill recalled Saturday. “She pulled off the wig and said, ‘Mike you have to hide me. They’re trying to get me out of the country.’ ”

Ms. Adams did leave Nepal for a time, but she managed to return. After the country’s monarchy was abolished in 2008, she became one of the few outsiders granted ­Nepali citizenship.

She wrote a regular newspaper column, called “Barbara’s Beat,” which she later collected and published in book form. All the while, she continued to dash around Kathmandu in her Sunbeam sports car, her now-white hair flowing in the wind.

In recent years, Ms. Adams began to deplore the overpopulation and environmental degradation that had overtaken parts of Nepal. She spoke out against political corruption, earning the enmity of the government that had replaced the old monarchy.

In 2011, she launched the Barbara Peace Foundation, which has built dozens of houses for Nepal’s poorest people. Well into her 80s, Ms. Adams traveled to remote parts of the country, coordinating efforts to improve housing and education.

In April 2015, two months after Ms. Adams had a near-fatal heart attack, Nepal was struck by a devastating earthquake. Six men carried her down 13 flights of stairs at a hospital to reach safety.

She returned to her Kathmandu home, surrounded by gardens and flowers.

“This is my home,” she said in a 1990 interview with the Los Angeles Times. “I had always planned to die here, and, well, my plans remain unchanged.”