NEW YORK, NY.- Alice Mason, a real estate broker and hostess whose talent at social engineering reworked the populations of Manhattan’s most restrictive co-ops — the tony apartment buildings that lined Park and Fifth Avenues — and for a time altered the nightlife of what used to be known as New York society, died Jan. 4 at her home in Manhattan. She was 100.

Her daughter, Dominique Richard, announced her death.

When she was a young woman in the 1950s, Mason taught dance — rumba, salsa, cha-cha — and her earliest real estate clients were actors, including Marilyn Monroe and Rex Harrison. But when she met Alfred Gwynne Vanderbilt, a horsy scion of the railroad clan, she found herself unable to place him in certain buildings.

Vanderbilt money, as it happened, was too new for certain communities in the 1950s. And so she began to study the peculiar social structure of the Manhattan cooperative. (She eventually found Vanderbilt a penthouse on East 79th Street.) By the 1980s, the Reagan years, she was a master at the game, running her own firm, Alice F. Mason Ltd., when even newer money was ascendant and the leveraged-buyout kings and their wives needed help passing muster with the gatekeepers of exclusive buildings such as 740 Park, once home to the Rockefellers and the Bouviers.

“The 1980s were the last gasp of New York’s old hierarchy, when the oldest and the newest richest families traded the palatial apartments that symbolized the underlying continuity of the city’s evolving elite,” Michael Gross, whose 2005 book, “740 Park: The Story of the World’s Richest Apartment Building,” mapped that evolution, said by email. “Alice was more matchmaker than real estate broker. She knew the buildings, the co-op boards and the buyers and greased the wheels for all concerned.”

For the fashion mogul hoping to buy a $10 million apartment in a Fifth Avenue co-op, Mason’s instruction was to donate $10 million to the Metropolitan Museum of Art, because the co-op board president was a member of the Met’s board. To prepare a Saudi prince for his co-op board interview, she tricked him into thinking that the event was a cocktail party in his honor, knowing his pride would never accept the real purpose, which was to let his potential neighbors look him over. For the wife of a furrier who had grown up in the outer boroughs and had an accent to match, Mason’s suggestion was to pretend to have a cold and stay mum and let her husband do the talking during the board interview.

And then there were the dinners, black-tie affairs in her elegant Upper East Side apartment to which Mason summoned the city’s power brokers, a fizzy mix of moguls, journalists and authors, diplomats and heads of state. The evenings were strictly choreographed: 60 guests seated at eight small tables strewed throughout her apartment and sized so that guests had to pursue a single topic together, rather than chat in pairs. Mason disliked small talk and discouraged it among her guests.

There were regulars like Norman Mailer, the burly author, and his wife at the time, Norris Church Mailer; Helen Gurley Brown, the longtime editor of Cosmopolitan, and her husband, producer David Brown; Gloria Vanderbilt and Barbara Walters; and Aileen Mehle, otherwise known as Suzy, the syndicated gossip columnist, who noted the nights’ doings in her column. As Mason told The New York Times in 1982, “I really prefer achievers because I’m interested in world affairs and politics, not small talk.”

The editor Tina Brown found the dinners addictive, as she wrote in “The Vanity Fair Diaries,” her 2017 memoir of her years running that magazine, even as she complained about the food. She found herself rapt on one typical evening — “a pop-up book of Reagan-era money,” she called it — as Carl Icahn, the corporate raider who had just stripped the assets of TWA, argued with Malcolm Forbes about the morality of corporate takeovers, their voices rising alarmingly until Norman Lear broke in over the shouting, declaring that he was going to take over the conversation.

President Ronald Reagan’s tax reforms were the topic on a night in 1985 when, as Charlotte Curtis reported for the Times, Alexander Haig was the guest of honor. Laurence Tisch, head of Loews, the hotel chain, was worried about a recession, and a teenage “Princess Chantal,” Curtis wrote, “who would be queen of France if the Bourbons had kept their heads, was merely perplexed. ‘It is all very confusing,’ she said politely, and nobody disagreed.”

Politicians and their fixers were always in the mix. Mason was an ardent Democratic fundraiser whose greatest political passion was Jimmy Carter, for whom she raised hundreds of thousands of dollars. Despite her no-nonsense mien, she believed in the occult practice of numerology, and Carter’s numbers were stellar, in her reading. She predicted his presidential win in 1976 and, to her despair, his loss four years later to Reagan.

Mason held nine dinners each year. Many grumbled when they were summoned, but almost no one declined an invitation.

“It was all part of the world-weary affect of elite New Yorkers to profess boredom with the scurrying of New York society while being absolutely determined not to miss a single second of it,” said English journalist Christopher Mason (who was not a relation). To his surprise, he became a regular.

“I was by far always the least powerful person there,” he said by phone, “but I had nonstop anecdotes, so I was a reliable dinner partner.”

One night stood out for him. He was seated in the den, at a table with Claus von Bulow, the Danish-born man about town who had been acquitted, on appeal, of trying to murder his wife, the heiress Sunny von Bulow; Philip Johnson, the modernist architect; and Agnes Gund, the arts patron. “‘Isn’t New York extraordinary?’” Christopher Mason recalled von Bulow saying. “‘Isn’t it absurd? What are we all doing here, dressed in black tie at the home of a Realtor?’”

It was Johnson who answered: “Oh, it’s very simple. I’m here because I’m a famous architect. Christopher’s here because he writes about famous people. Aggie is the president of MoMA. And you’re here because you’re a famous murderer.” There was a pause, Mason remembered, and then von Bulow roared with laughter. The conversation moved on.

Alice Mason was a social arbiter, but she was not social. Her pleasures were work and high-stakes gin rummy. In between commissions, she lived on her card winnings. She preferred the single life, although she tried marriage three times. Her first marriage, to a distant cousin, lasted six months. Her second, to Francis Richard, a Frenchman who had moved to New York to open a Berlitz language school, and with whom she had her daughter, Dominique, lasted three years. Her third, to Jan Schumacher, a Dutch diplomat, was the briefest, at three months.

“I really considered marriage a very boring thing,” she told New York magazine in 1984. “I mean, I don’t think companionship is that marvelous. I never feel lonely.”

For most of her working life, Mason also had a secret: She was a Black woman passing as white. Even her name was a fiction.

She was born Alice Christmas on Oct. 26, 1923, in Philadelphia “to a bourgeois family of color,” as she wrote in an unpublished memoir. Her father, Lawrence Duke Christmas, was a dentist; her mother, Alice (Meyers) Christmas, managed the household. In Alice’s telling, the family was so light-skinned they were known as the White Christmases.

Yet Alice’s world was circumscribed and sheltered, she said, and she didn’t interact with any white people until she attended Colby College, in Maine. It was her race-conscious mother who decided that Alice should “pass” and live her life in the white world, so as not to face the era’s prejudices toward people of color. Her mother arranged a marriage with a light-skinned cousin named Joe Christmas. Joe was not keen on passing, however, and Alice was not keen on marriage — hence the divorce. By the late 1940s, she had moved to New York City, knowing no one there.

She soon christened herself Alice F. Mason. She liked the actor James Mason, and the F stood for Fluffy, an incongruous nickname given to her by Vanderbilt because she was anything but. It was also a potent combination of letters, in numerology terms.

Mason’s secret came out in 1999, when her family ties were noted in “Our Kind of People: Inside America’s Black Upper Class,” by Lawrence Otis Graham. But it was hardly a bombshell. No one seemed to notice or care. “There are many people with family members who live on both sides,” Mason told New York magazine. “I’ve led this life for over 45 years, and it’s all a state of mind.”

In addition to her daughter, she is survived by a grandson.

Mason closed her firm in 2009, when she was 86. The rich no longer needed her specialized knowledge. Park Avenue, with its fussy, archaic rules, had ceded much of its cachet to the glassy new condos being built downtown, to which the ticket for entry was simply money.

She never left the rent-stabilized apartment where she held her storied dinners, in a century-old building on East 72nd Street. (In Manhattan real estate parlance, it was a classic eight, a gracious prewar layout that included three bedrooms and two maid’s rooms.) In 2011, developer Harry Macklowe bought the building for a reported $70 million and began to turn the units into condos, buying out the tenants to do so.

But Mason refused to give up her apartment. When she moved there in 1962, the rent was $400 a month. At her death, it was $2,476. The apartment below her, in the same line, was recently on the market for just under $10 million.

This article originally appeared in The New York Times.