Auchincloss died at a New York hospital on Tuesday, a week after suffering a stroke, said a grandson, James Auchincloss.
World War II, and crafted such accomplished works as the novel "The Rector of Justin" and the memoir "A Writer's Capital." He also wrote biographies, literary criticism and short stories. He was a four-time fiction finalist for the National Book Award, his nominated novels including "The Embezzler" and "The House of Five Talents."
Living up to the Old World ideal of being "useful," Auchincloss bore the various titles of writer, attorney, community leader and family man. He was a partner at the Wall Street law firm of Hawkins, Delafield & Wood and the father of three. He served as president of both the Museum of the City of New York and the American Academy of Arts and Letters.
He was also a cousin by marriage to Jacqueline Kennedy Onassis, and she worked with him when she was a book editor late in her life.
Auchincloss had an old-fashioned aversion to change, and his presence could be likened to a building granted landmark status. Into his 90s, his stride was erect, his jawline stubbornly firm, his dark, moody eyes less windows to the soul than the most discriminating of gatekeepers.
For subject matter, he followed the advice Henry James once offered to Edith Wharton: "Do New York. The firsthand account is precious." Auchincloss documented an exclusive, influential world of which he was both member and critic. Readers were taken into boardrooms, country clubs, summer homes at Bar Harbor, dinner parties on Fifth Avenue.
This led to both praise and criticism, sometimes with the same ambiguous compliment, "America's foremost novelist of manners." It was not unlike being called "America's premier polo player" or "America's finest maker of top hats." In 1970, critic Granville Hicks derided Auchincloss' "little world," saying it had nothing to do with the rest of society.
In response, Gore Vidal defended Auchincloss for informing readers about "the role of money in our lives" and showing "how generations of lawyers have kept intact the great fortunes of the last century."
On Wednesday, Vidal praised Auchincloss as a close friend whose literary subject was unique. "Nobody else took those kinds of people, because nobody else understood them, except in the dumbest way," Vidal said.
Auchincloss did not seek to praise the rich but to make sense of them. He was a Puritan who brooded over the conflicts between money and principles, over the privileged person's right to be happy. Several of his best-known books featured introspective young men and women.
"How can you live in the times we live in and not feel, 'Doesn't there have to be some evening up?' 'Why should I have this wonderful life when those people went through what they did in Germany?' " Auchincloss said.
He was born in the town of Lawrence on New York's Long Island in 1917, his ancestors having arrived from Scotland more than a century before. Auchincloss' grandmother knew Edith Wharton and his father was a Wall Street lawyer. The family and their servants lived in a Manhattan brownstone a block from the Park Avenue residence where Louis Auchincloss later lived.
His early years were typical of his class: a Fifth Avenue private school, Groton prep school, Yale University, law school at the University of Virginia. After serving in the Navy during World War II, he completed his first novel, "The Indifferent Children," a war story the insecure young writer published under the pen name Andrew Lee.
Reviewers praised the book, however, and from that point, the author remained forever Auchincloss. He quit law practice in 1951, discovered the additional time did nothing for his writing and returned to it in 1954.
His other works included the novels "Her Infinite Variety" and "Pursuit of the Prodigal," the story collection "Tales of Manhattan" and a short biography of Theodore Roosevelt for Times Books' presidential series.
Auchincloss' wife, Adele, an artist and environmentalist, died in 1991. Survivors include three sons, a brother and seven grandchildren.