Home during the winter was the 115-room Mar-a-Lago estate, which was bought by Donald J. Trump in 1985 and converted into a private club. (Mr. Trump’s daughter Ivanka Trump slept in the same children’s suite that Ms. Merrill had used.)
As a child, born into the American aristocracy of money and high society, Ms. Merrill wished she could take the bus “like the other kids,” she said, instead of being driven to school by the family chauffeur. After she became a successful actress, she told Quest magazine, “It’s fascinating to lead someone else’s life for a while.”
But as it turned out, the “someone else” was almost always a coolly sophisticated patrician woman not that different from the real Dina Merrill. Typical of her parts, in the 1959 television version of Budd Schulberg’s “What Makes Sammy Run?” she was the glamorous daughter of a Wall Street banker.
Although her father’s investments had earned her a million dollars by the time she became an actress, against his wishes, Ms. Merrill supported herself by modeling clothes for Vogue at $10 an hour.
“It never occurred to me to ask my father or mother to pay for something they didn’t believe in,” she said in a 1979 interview. “My ambitions were my own — not exactly the ones they had for me.”
Her father wanted her to become a lawyer and then to run for Congress. Instead, Ms. Merrill made her Broadway debut — speaking three lines — in John Van Druten’s “The Mermaids Singing” in 1945.
She was born Nedenia Marjorie Hutton on Dec. 9, 1923, in New York City and nicknamed Deenie. Her parents divorced when she was 10.
She attended George Washington University, but dropped out after a year to study at the American Academy of Dramatic Arts in New York. In 1946 she married Stanley M. Rumbough Jr., heir to the Colgate-Palmolive consumer products fortune, and spent much of the next decade raising their three children. By the time she got her first movie role — as a young research assistant to Katharine Hepburn in “Desk Set” (1957), with Spencer Tracy — she was over 30.
Her subsequent roles included the bored upper-class wife of an Australian sheep rancher in the Deborah Kerr movie “The Sundowners” (1960), and the alcoholic wife of an entrepreneur played by the comedian Alan King in “Just Tell Me What You Want” (1980).
She also had a thriving career as a guest star on television series, including “Bonanza,” “Mission: Impossible” and “Murder, She Wrote,” and as an actress in made-for-TV movies.
She returned to Broadway in 1975, starring as a wife whose husband is trying to drive her mad in a revival of the play “Angel Street.” In 1983 she played the manager of the Russian Ballet in a well-received Broadway revival of the Rodgers and Hart musical “On Your Toes.”
Divorced from Mr. Rumbough and married to the actor Cliff Robertson in 1966, she was partly responsible for bringing down the head of a Hollywood studio. When David Begelman, the president of Columbia Pictures, embezzled $10,000 by forging Mr. Robertson’s name to a check, no one paid much attention, Ms. Merrill said, until she called her friend Katharine Graham, the publisher of The Washington Post.
“Cliff took the telephone and told the whole story,” she recalled. “Kay put an investigative reporter on it, and then it really became public.”
With an inheritance from her parents estimated at more than $50 million, Ms. Merrill became a philanthropist. A liberal Republican, she was vice chairwoman of the Republican Pro-Choice Coalition, an advocate on women’s health issues and vice president of the New York Mission Society. After her son David, who had diabetes, died in a boating accident at age 23 in 1973, Ms. Merrill created a yearly award for scientific excellence in his name for the Juvenile Diabetes Research Foundation.
After divorcing Mr. Robertson in 1986, Ms. Merrill married Ted Hartley, a former Navy fighter pilot, actor and investment banker, who survives her. Shortly after their marriage in 1989, their company, Pavilion Communications, bought a controlling interest in RKO Pictures, but they had little success in returning that studio to its former glory.
In addition to her son and her husband, Ms. Merrill is survived by a daughter from her first marriage, Nina Rumbough Roosenburg; a stepson, Philippe Hartley; six grandchildren; four stepgrandchildren; and two stepgreat-grandchildren. Heather Robertson, her daughter with Mr. Robertson, died of cancer in 2007. Mr. Robertson died in 2011.
Ms. Merrill had some regrets about her late-blooming acting career, which had been forestalled because of her child-rearing responsibilities.
“You didn’t go to work then if you had young children,” she said in 1979. “But the 20s are very important years to an actress. If I had it to do over again today, I might continue working.”