For more than half a century, the art collector Hester Diamond called the imposing, twin-towered Eldorado on the Upper West Side home. It was there, in her sprawling duplex, that she put together extensive collections of Modernist and old masters works, entertained art world dignitaries and raised her family.

“My parents didn’t separate their social lives and work lives — they existed in the same ecosystem in the apartment,” recalled one of her sons, Michael Diamond, known as Mike D, one of the founders of the Beastie Boys.

Ms. Diamond and her first husband, Harold Diamond, started their collections slowly. “They loved art and going to galleries,” said their oldest son, David Diamond, a marketing and strategic planning consultant. “They bought extensively on the layaway plan — my mom used to joke that they owed $50 a month to every gallery in town.” Eventually they gave up their day jobs — she was a social worker, he a schoolteacher — to pursue careers in art, then set up their own business.

Growing up at 300 Central Park West, the Diamond brothers became art experts in their own right. “When I was a kid, my friends would say I lived in a museum,” said David Diamond, 63. “But somehow the family felt completely comfortable with it.”

Since Ms. Diamond’s death two years ago at age 91, all of the art and furnishings have been auctioned off, bringing in more than $26 million, as well as donated or divided among family members. And now the empty duplex is up for sale. The asking price is $19.5 million, with $22,200 in monthly maintenance, according to the broker Amy Katcher of the Corcoran Group, who is listing the property with Michael J. Franco of Compass.

The apartment sits on the 18th and 19th floors of the south tower. It measures around 6,300 square feet and includes a total of around 800 square feet of terraces on both levels, offering stunning vistas of the Central Park Reservoir and Midtown skyline. “The views, the light and the space are incredible,” Ms. Katcher said.

The Diamonds came to the Eldorado as renters in the mid-1960s. After the 1929 Art-Deco complex was converted into a co-op in 1982, they bought the duplex, along with an adjoining apartment on the 19th floor. (David Diamond believes his parents paid around $1.2 million for both units.) In November 1982, with renovations still ongoing, Mr. Diamond died at age 56. (A third son, Stephen, died in 1999.)

“The renovation was completed in the summer of 1983, and the apartment was fully put back together in the fall,” David Diamond said. “But by then, it was clear that the renovated apartment was really my mom’s apartment. It was, in a real sense, the start of her new life.”

The living space was further enlarged in 2009, when Ms. Diamond acquired another adjacent apartment, with two bedrooms and two bathrooms, on the 18th floor.

The home is being listed with six bedrooms and seven and a half bathrooms, though the configurations have shifted numerous times over the years. “Pretty much every room in the house changed its function at least once during the 50 years my mom lived there,” Michael Diamond, 56, said.

Ms. Diamond, who also worked as an interior designer in addition to dealing in art, liked to shake up the symmetry of the apartment as well. She was known in the art world for mixing classic English furniture and antiques with bold Modernist art, then years later switching to bright, contemporary furnishings juxtaposed with works of the old masters.

Her vast collections of paintings, sculptures and other memorabilia were displayed throughout the home and included pieces from Fernand Léger, Pablo Picasso and Pietro and Gian Lorenzo Bernini.

“I loved her fearlessness and her vision that she had, to switch so dramatically and radically,” Michael Diamond said. (Both he and his brother say they preferred the contemporary art over the old masters, largely because it was what they grew up with.)

The main entrance to the apartment is on the 19th floor. A spacious foyer leads to a study and a 29-by-22-foot living room, each opening to a wraparound terrace. Off the foyer and through a long gallery are a large, eat-in kitchen with stainless-steel cabinetry, a pantry and professional-grade appliances; a formal dining room with an expanded china closet; and an enormous primary bedroom suite, which features a terrace, a full bathroom, powder room and an sizable dressing room.

On the lower level, connected by a curved staircase, are the remaining bedrooms, two of which have terrace access, and bathrooms. More recently they had been used as a home office, fitness room, library and music room. There is also a laundry room and staff quarters.

Throughout the apartment are many original architectural elements, like the high ceilings, herringbone oak floors, plaster moldings and oversize windows. Central air-conditioning and dehumidification and lighting systems have also been added.

Michael and David Diamond say they have fond memories of growing up in the Eldorado, located between 90th and 91st Streets. The family frequented nearby museums and spent countless hours in the park.

“It really was our backyard,” said Michael Diamond, adding that he would often skateboard there, as well as play baseball and soccer.

Ms. Diamond especially enjoyed spending time in the apartment’s grand living room, the brothers said, where she would often entertain a wide range of guests as well as just sit and relax.

“When she wasn’t entertaining,” Michael Diamond said, “she was in her corner office,” proudly adorned with Beastie Boys gold records.

She was probably his biggest fan, but he admits that she was less of a fan of his music.

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