Environmentalist, novelist and wildlife author best known for The Snow Leopard
Although he saw himself primarily as a novelist, Peter Matthiessen, who has died of leukaemia aged 86, became best known for his non-fiction writing, a phenomenon he once described as "being pushed so far into a pigeonhole I now doubt I will ever get out". Indeed, Matthiessen's non-fiction earned him an important place among conservationists worldwide. His writing encompassed nature and travel, and its spiritual insights about nature, man, and himself, turned him into a sort of new-age guru. Even the careful craftsmanship of his fiction reflected an approach to writing that echoed the Zen Buddhism he practised.
Matthiessen was born into a privileged background, in New York, where his father Erard was a successful architect, who during the second world war designed systems to protect transatlantic merchant convoys, and later became an executive of the Audubon Society, an environmental association. The family had houses on Long Island and in Connecticut, and Matthiessen was educated at St Bernard's school, Manhattan, and Hotchkiss school, in Lakeville, Connecticut. At Yale University, he studied English and spent a year at the Sorbonne in Paris, where he met Patsy Southgate, who would become his first wife. After graduating in 1950, he stayed at Yale to teach.
One of his undergraduate stories won a prize from the Atlantic Monthly magazine but further success eluded him, and having been recruited, like many well-connected Yale old boys, by the CIA, he returned with Patsy to Paris. There, along with other struggling writers including George Plimpton, he founded the Paris Review, and created its most notable feature, the extended Writers at Work interviews. The CIA at this time was supporting a number of journals overseas, most notably Encounter, though Matthiessen later insisted that his function had been merely to keep an eye on "suspicious" expatriates.
On his return to New York, his first novel, Race Rock (1954), was published, followed by Partisans (1955), set in Paris. By now Matthiessen had settled in Sag Harbor, on Long Island, and supported his family by running fishing charters in the summers. In the wake of his divorce, he turned to non-fiction, transforming a series of articles written for Sports Illustrated into the successful book Wildlife in America (1959). He had a short story adapted into a film, The Young One (1960), by Luis Buñuel, but his third novel, Raditzer (1960), about the scion of a wealthy family going to sea, made little impact.
However, his journalism led him to the door of the New Yorker, and its editor, William Shawn, encouraged him to travel farther afield. The resulting books, Cloud Forest (1961), set in the Amazon and Tierra del Fuego, and Under the Mountain Wall (1962) in New Guinea, established his reputation.
In 1963, he married Deborah Love, a writer who introduced him to Zen Buddhism. Together they acted as "guinea pigs" in early LSD experiments, before the drug was outlawed. They settled on a six-acre estate in Sagaponack, Long Island, which became a summer meeting place for writers from New York and a venue for Zen retreats.
In 1965 he published At Play in the Fields of the Lord, a novel about fundamentalist missionaries in the Amazon, which began to establish the powerful synthesis of the two strands of his writing. It became a film in 1991, directed by the Brazilian Héctor Babenco.
It would be 10 years before his next novel, but in that time Matthiessen published five non-fiction works, including The Shorebirds of North America (1967) and Blue Meridian: In Search of the Great White Shark (1971), which chronicled the making of a documentary documentary about sharks, Blue Water, White Death. When he returned to fiction, it was with Far Tortuga (1975), set among turtle hunters in the Cayman Islands, whose impressionistic style James Dickey called "a turning point in the evolution of the novel".
Three years later, he won two National Book awards for a non-fiction account, The Snow Leopard (1978), in which the search for the animal in the Himalayas, his wife's death from cancer and his quest for Zen awareness were interwoven to create a seamless meditation.
Matthiessen had moved politically to the left, opposing the Vietnam war and becoming involved with the farm union leader Cesar Chavez, the subject of his 1969 book Sal Si Puedes. In the Spirit of Crazy Horse (1983) delved into the history of the Sioux nation and its relationship with the US, while arguing the innocence of the Lakota leader Leonard Peltier, convicted of killing two FBI agents in a South Dakota siege in 1975. Matthiessen and the book's publishers, Viking Press, faced libel lawsuits from one agent and the former governor of South Dakota, which caused the book's withdrawal. Although the suits were eventually dismissed, the legal costs went over $2m. A paperback edition of the book was finally published in 1992.
In 1980 Matthiessen married Maria Eckhart. Over the next 20 years he would publish eight more non-fiction works, including the elegiacal Men's Lives (1986), about fishermen in South Fork, Long Island, and reminiscent of his New Yorker predecessor Joseph Mitchell's At the Bottom of the Harbour. During this time, he also worked on a trilogy of novels based on the life of EJ Watson, a plantation owner who nearly ruled the Ten Thousand Islands section of the Florida Everglades. Killing Mister Watson (1991), Lost Man's River (1997) and Bone By Bone (1999) were remarkable works that drew on both classic American literature and elements of hard-boiled detective fiction. In 2008 he won another National Book award for Shadow Country, in which he reshaped the trilogy as one novel.
Active until the end, Matthiessen published two remarkable late works, Birds of Heaven (2001), about the search for cranes, and End of the Earth: Voyages to Antarctica (2003). A final novel, In Paradise, about a Zen retreat held on the site of a Nazi death camp, is due to be published later this week.
He is survived by Maria; a son, Luke, and daughter, Sara, from his first marriage; and a son, Alex, and daughter, Rue, from his second.
• Peter Matthiessen, writer and environmentalist, born 22 May 1927; died 5 April 2014