Peter Matthiessen, whose life of ad­ven­ture and spiritual questing is woven throughout his many acclaimed books, including “The Snow Leopard,” “At Play in the Fields of the Lord” and a trilogy of novels set in the wilds of Florida, died April 5 at his home in Sagaponack, N.Y. He was 86.

His publisher, Riverhead Books, announced the death. He had been diagnosed with leukemia more than a year ago.

In a varied career, Mr. Matthiessen worked for the CIA, founded the influential Paris Review literary journal, was a commercial fisherman on Long Island and traveled to remote regions of the world on assignment for the New Yorker magazine. He also became a Zen master, and a defender of the environment and the cause of the American Indian.

From the start, Mr. Matthiessen struggled against the privilege and pedigree he had been born into. He was educated at the finest preparatory schools and graduated from Yale University, but, at 15, he demanded that his name be removed from the Social Register.

Later, he rode tramp steamers, canoed through jungles, hiked across the Himalayas and practiced meditation in a lifelong personal journey that was reflected in more than 30 books.

Peter Matthiessen in 1978. (Henry Allen/The Washington Post)

He won the National Book Award for “The Snow Leopard,” a 1978 account of a spiritual trek through the mountains of Nepal, and for “Shadow Country,” a 2007 reworking of a trilogy of novels set in the remote Ten Thousand Islands of Florida, in which an overbearing plantation owner is gunned down by a posse of neighbors.

Mr. Matthiessen was the only writer to have won the National Book Award in both fiction and nonfiction. His final novel, “In Paradise,” is being published this week.

Besides his books, one of Mr. Matthiessen’s most lasting literary contributions is the Paris Review, which he founded with Harold L. “Doc” Humes in Paris in the early 1950s. Mr. Matthiessen chose a childhood friend, George Plimpton, as the editor of the journal, which publishes fiction and long-running series of in-depth interviews with writers.

Decades later, it was revealed that Mr. Matthiessen was working for the CIA when he launched the Paris Review.

“I needed more cover for my nefarious activities,” he said in an interview for “George Being George,” an oral biography of Plimpton, who died in 2003. Mr. Matthiessen said that the CIA had nothing to do with the content of the magazine, but Plimpton was “shocked and very angry” when he learned the truth.

“Who, after all,” Mr. Matthiessen said, “wants to hear that the ‘love of his life,’ as he himself would call it, had been conceived as a cover for another man’s secret activities?”

Mr. Matthiessen left Paris and the CIA in 1953, settled on Long Island, worked as a commercial fisherman and published three early novels. In 1956, he loaded his Ford convertible with books, a sleeping bag and a shotgun and visited every nature preserve in the United States.

He produced his first nonfiction book, “Wildlife in America,” in 1959, which led to a new career as a chronicler of the natural world for the New Yorker. His nonfiction accounts of wildlife and native peoples in South America, New Guinea, Siberia and East Africa were credited with drawing attention to environmental concerns.

“The Snow Leopard,” which became his first bestseller, combined a spiritual autobiography with a scientific expedition’s search of the elusive mountain cat of the Himalayas. Critics praised Mr. Matthiessen for his personal, observant, casually polished prose style, as he ventured into an exotic, little-known world:

“Even in rain, this landscape is hallucinatory — gorges and waterfalls, the pines and clouds that come and go, fire-colored dwellings painted with odd flowers and bizarre designs, the cloud-mirrors of the rice paddies in steps down the steep mountainside, a flock of vermilion [birds], blown through a wind-tossed tumult of bamboo.”

Mr. Matthiessen also wrote two books about what he considered injustice toward American Indians. The most controversial was “In the Spirit of Crazy Horse” (1983), which lambasted the federal government’s treatment of American Indians and championed the innocence of Leonard Peltier, a member of the American Indian Movement who was convicted of killing two FBI agents in 1975 at Pine Ridge Reservation in South Dakota.

Mr. Matthiessen was sued for libel by an FBI agent and then-South Dakota Gov. William Janklow. After six years, the suits were dismissed, but Peltier remains in prison.

Despite the high profile of his nonfiction, Mr. Matthiessen considered fiction a higher artistic calling. His 1965 novel, “At Play in the Fields of the Lord,” about the struggles faced by missionaries in Brazil, was made into a 1991 film. “Far Tortuga,” a 1975 novel about turtle hunters written entirely in a Caribbean patois, met with mixed reviews.

Mr. Matthiessen made his biggest fictional breakthrough in 1990 with “Killing Mister Watson,” set in the wilds of southwestern Florida at the turn of the 20th century. The central character, Edgar J. Watson, a real-life figure who was charismatic and psychopathic in equal measures, was shot and killed by at least 20 of his neighbors in 1910.

Mr. Matthiessen told his story in the voices of various characters, including that of Watson himself.

The book was praised as an American counterpart to Joseph Conrad’s “Heart of Darkness.” In the Los Angeles Times, novelist Marianne Wiggins wrote that “Killing Mister Watson” “stands with the best that our nation has produced as literature.”

After two sequels, “Lost Man’s River” (1997) and “Bone by Bone” (1999), Mr. Matthiessen condensed the three books into a single volume, “Shadow Country,” which brought him a second National Book Award.

Peter Matthiessen was born May 22, 1927, in New York City. His father, Erard, was an architect and conservationist who became a leader in the National Audubon Society. The younger Matthiessen grew up primarily in Connecticut, where he attended the private Hotchkiss School.

After serving in the Navy at the end of World War II, Mr. Matthiessen graduated from Yale in 1950. He had spent a year at the Sorbonne in Paris in college and returned to France in 1951 as a CIA officer. Working for the espionage agency, he later said, offered “a free trip to Paris to write my novel.”

In the 1960s, Mr. Matthiessen experimented with LSD and mescaline and began to practice Zen Buddhism, which became an increasingly important part of his life. He spent several weeks a year at Zen retreats, meditating in cross-legged silence for up to 14 hours a day and eventually led daily meditation sessions at his home.

His first marriage, to Patsy Southgate, ended in divorce. His second wife, Deborah Love, died of cancer in 1972. Survivors include Maria Eckhart, his wife since 1980; two children from each of his first two marriages; two stepchildren; and six grandchildren.

Despite his formidable reputation as a nonfiction author, Mr. Matthiessen considered himself primarily a novelist.

“I’d like to think that in 100 years, I’ll be more remembered for my fiction,” he told Newsday in 1997. “I just find fiction much more energizing. . . . Nonfiction [is] like cabinet-making. It can be very beautiful. But that’s not the same as sculpture.”