Napa's Grande Dame

It was 2009 and Mrs. Mondavi's husband, famed vintner Robert Mondavi, had died the year before. After 25 years of living in a storied Napa Valley estate, in an 11,500-square-foot home best known for the swimming pool in its living room, she was ready to downsize.

The Ruin was the first house her real-estate agent showed her. Mrs. Mondavi recalls the 3,000-square-foot, four-bedroom, five-bathroom house as being dark, with a roof that leaked and walls that blocked the views. That was before she looked at 25 other listings, which she describes as "horrors"; she decided to return to the Ruin a second time. On her third visit, she brought along friend and architect Howard Backen to ask his opinion of the house and 9 acres that sit off a narrow, winding road on a hillside overlooking the Napa Valley.

"He said 'Take it, I'll do it, you'll never get a view like that,' " she remembers. And so she took it, on the condition that Mr. Backen, whose clients have included Robert Redford, Jeffrey Katzenberg, Steve Jobs and numerous Napa wineries, would design the house.

"Margrit said, 'Can you make this beautiful?' and I said, 'You can make anything beautiful, but—' and she turned to the real-estate lady and said, 'I'll take it because he can make it beautiful.' I had a 'but' there, but she cut me off. I didn't finish the sentence and then I was committed," he says.

Mr. Backen can now elaborate on his "but": The windows were wrong. The space was too broken up. There was no connection between the house and the view or the outdoor space. Mrs. Mondavi wanted a pool off of her bedroom, but that meant building it 15 feet off the ground.

Originally from Switzerland, Mrs. Mondavi met her future husband in 1967 while working as a tour guide at the winery that he founded in 1966 and turned into an iconic brand. She went on to become director of public relations, starting programs such as the winery's summer music festival, which has brought artists like Ella Fitzgerald and Harry Belafonte to Napa.

Mr. and Mrs. Mondavi were married for 28 years and beyond the winery—which Constellation Brands acquired in 2004—were well-known philanthropists, donating $35 million to the University of California at Davis to establish the Robert and Margrit Mondavi Center for the Performing Arts and the Robert Mondavi Institute for Wine and Food Science. Their Wappo Hill home was known as a place that brought together a who's who of wine, food and art.

Wappo Hill, which was originally asking $25 million, sold for an undisclosed amount in 2011 to Jean-Charles Boisset, proprietor of Boisset Family Estates, which owns a collection of wineries in France and California, and his wife, Gina Gallo, a winemaker at her family's E. & J. Gallo Winery. Mr. Boisset says they weren't just attracted to the architecture of the Cliff May-designed home, but also the property's history, oil painting reproduction.

Mrs. Mondavi declines to say what she paid for her home or how much she spent on the renovation. Agi Smith, a real-estate agent with Pacific Union International-Christie's International Real Estate, who saw the property before it was redone, says a home like this would go for at least $4.5 million. Nearby, a 5,000-square-foot home on 5? acres with a pool is listed for $6 million.

If Mrs. Mondavi was ready to downsize, it doesn't mean she was ready to slow down. She starts each day with a 50-minute workout in the solar-heated, salt water infinity pool just off the master bedroom, as she had envisioned. She still regularly goes to her office at the winery some 35 minutes away in her convertible.

With two bedrooms and 2? baths, the modern, one-level house has been kept simple. The floor is a neutral travertine. The walls and ceilings are Backen White (Mr. Backen's personal formula, which he describes as a "warm white"). The entrance hall disappeared to enlarge the living room.

Mr. Backen eliminated as many walls as possible. On the west side of the house, facing the view of the valley, he added walls of glass doors out onto the terrace. On the east side of the house, glass doors open onto the garden.

To create a connection between indoors and out, landscape designer Claudia Schmidt and Mr. Backen created a "Zen garden" off the dining room, which includes a seating area and a dining area on a raised terrace. The Chinese pistache tree shades the area from the sun and Ms. Schmidt added a small fountain. It is now a favorite spot of Mrs. Mondavi's and is easily accessible from the kitchen, where she spends much of her time.

Ms. Schmidt also created the "laundry garden." The house came with a clothesline that Mrs. Mondavi wanted to keep (she uses it), but Ms. Schmidt surrounded it with Mrs. Mondavi's cutting and vegetable garden. Mrs. Mondavi chose the cutting roses. As with the interiors, Ms. Schmidt describes the palette of the garden as "subtle and calming" with shades of gray, green, white, purple and blue throughout.

Mrs. Mondavi replaced the lawn surrounding the entrance with gravel and olive trees. Nestled there are 90 stone mushrooms, eight to 12 inches tall, made by a local artist. She gave them to her husband for his 90th birthday. "What are you going to give a man who has everything?" she asks.

For the interiors, Mrs. Mondavi brought in Thomas Bartlett, a designer in Napa who has known her for more than 40 years and advised on her previous home. They used much of the furniture from that home, including the living room sofas that Mrs. Mondavi had made up 18 years ago in heavy, cream colored cotton. "She had virtually everything and I spent a number of days pushing and pulling until everything felt right," says Mr. Bartlett.

For the interiors, Mrs. Mondavi brought in Thomas Bartlett, a designer in Napa who has known her for more than 40 years and advised on her previous home. They used much of the furniture from that home, including the living room sofas that Mrs. Mondavi had made up 18 years ago in heavy, cream colored cotton. "She had virtually everything and I spent a number of days pushing and pulling until everything felt right," says Mr. Bartlett.

Echoing the home's neutral palette of fabrics, Mrs. Mondavi's closet also tends toward muted colors. "I don't look good anymore in prints. I wear black, brown, navy. Color ages you."

Inside the house, the neutral background serves as a gallery for works by Picasso, Giacometti, Diebenkorn, Frankenthaler, Tamayo, Oliveira and Mrs. Mondavi's friend Wayne Thiebaud. There are also oil lamps from Jerusalem, betel-nut boxes from Myanmar and masks from Africa. There are Russian icons, Roman glass and antiques from Laos and Morocco.

The heart of the home is the kitchen, where Mrs. Mondavi likes to paint so she can hear her soups cooking. She stands at the marble counter with her brushes, her watercolors and a large pad of watercolor paper. She used to do oils, but grew tired of carrying around all the accouterments that go with oil painting. Her dog, Luce, a Bichon Frise who has only one eye after an unfortunate run-in with a bigger dog, sleeps in a flower-shaped bed at the end of the counter.

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