Winfield House, which has been home to American ambassadors here since 1954, is a stately neo-Georgian mansion nestled in 12 ½ acres in the northwest corner of Regent’s Park. Originally built for Barbara Hutton, the Woolworth heiress, in the 1930s, it was sold to the United States government for $1 shortly after World War II. Its imposing facade has not always been appreciated by the British; in 1937, when it was still under construction, the venerable British journal The Architectural Review described the redbrick structure as “pinkly glowing like an open sore.”

But its interiors are about as grand as grand can get, and the art inside has sometimes been even grander. In 1969, when the publishing magnate Walter H. Annenberg moved in, he brought much of his own collection, including three Monets, two Gauguins, five Cézannes, two van Goghs, three Renoirs and a Toulouse-Lautrec.

Nowadays filling an American ambassador’s residence with European paintings would raise a few eyebrows; instead, the current occupants have gone American. In the house’s entrance two large red-and-orange canvases by Mark Rothko share space with sculptures by Martin Puryear and Claes Oldenburg. In the stairwell, an Ellsworth Kelly faces a Jasper Johns. And the state rooms beyond are filled with prime examples of other American masters, including Roy Lichtenstein, Louise Bourgeois and Philip Guston.

This showcase has been assembled by Ambassador Louis B. Susman, a lawyer, retired investment banker and longtime Democratic Party fund-raiser, and his wife, Marjorie.

“From the moment Louis got the appointment I thought about bringing American art to Winfield House,” said Ms. Susman, a petite woman with a Midwest twang. The couple, who are from Chicago, have been art lovers and collectors for decades. Until his appointment Mr. Susman served on the board of the Art Institute of Chicago, and Ms. Susman still sits on the board of the Museum of Contemporary Art there. (From 1991 until 2009 she was also a representative for Sotheby’s.)

When they moved to London in 2009, much of their art came with them. There is a delicate red-and-black watercolor by Brice Marden; a 1997 dark blue curved canvas by Mr. Kelly (who provided them with detailed installation instructions for the move) and two abstract paintings by Ad Reinhart. Bringing their own things made them feel at home, Ms. Susman said.

But with its 35 rooms Winfield House has a lot of space to fill. So even before she arrived Ms. Susman started talking to museums, dealers and collectors about loans. She also teamed up with the State Department’s Art in Embassies Program, which was created in the early 1960s to organize and facilitate loans to American outposts abroad, especially diplomatic residences. “We made a dream list of what we wanted,” Ms. Susman said. “We see this as a way of combining our passion for art with our new diplomatic role.” The previous ambassador, Robert H. Tuttle, and his wife, Maria, were also art collectors, and they too had contemporary art around Winfield House — works by de Kooning and Rothko and Mr. Kelly .

Ms. Susman has a mission. “There is a very active art world in the U.K.,” she said, “and we’re initiating outreach. We have this elegant home, and it’s a public place, and we want as many people to enjoy it as possible.”

The National Gallery of Art in Washington served up the two Rothkos; Agnes Gund, the New York collector and president emerita of the Museum of Modern Art, lent several works too, including another painting by Mr. Marden and one by Lichtenstein; and the Pace Gallery sent several things from its stable of artists, like a 1958 painting by Agnes Martin and a colorful chrome and steel sculpture from 2007 by John Chamberlain.

Some works were chosen for their subject matter and history. The Willem de Kooning Foundation lent Winfield House charcoal drawings of the Washington Monument; the Art Institute of Chicago sent over a bronze American flag that Jasper Johns made in 1960. “Leo Castelli gave one to J.F.K. when he was in the White House,” Ms. Susman said, adding that one of the Chamberlain sculptures fashioned from twisted and smashed up car parts, reflects “America’s fascination with the automobile.”

Some artists, like Mr. Johns, lent them works from his own collection like the 1998 painting “Untitled (Red, Yellow, Blue)” that hangs opposite the grand staircase. “It’s always been nice to have American art shown abroad,” Mr. Johns said.

As advocates for American art, the Susmans have not been shy in reaching out. In early 2010, when the actors Alfred Molina and Eddie Redmayne were playing in “Red,” the play about the life of Mark Rothko that ran in London before coming to New York, they were invited to come study the Rothkos in the entrance hall. The couple have also been host to gospel choirs and museum directors; other actors like James Earl Jones and the cast of “Cat on a Hot Tin Roof”; as well as scores of art students from places like the Courtauld Institute of Art and the Royal College of Art.

Two students from an art college in Birmingham, who are involved in a study program with the Ikon Gallery there, were among a group of 11 who toured Winfield House last year. “I thought it was incredible to see such important works out of a gallery context,” said Sean Burns, 19, who said that he was also “really impressed to see the Rothkos and other Abstract Expressionists” he had been studying. One glaring omission Mr. Burns noticed: “They didn’t have a Warhol,” something he said he mentioned to Ms. Susman.

His classmate James Lawrence Slattery said visiting Winfield House was particularly interesting because it gave her a chance to see a Jasper Johns for the first time. “In real life, up close, it has real impact,” she said.

For Ms. Susman, “the art tells an American story,” she said one wintry afternoon over tea in the Green Room, an elegant space that has walls covered in 18th-century Chinese wallpaper and French doors looking out to the garden. Over the fireplace, where a Gainsborough portrait once hung, a bright red abstract canvas by Reinhart makes for a somewhat startling centerpiece.

The couple say they are well aware that not everyone who visits will be pleased by their taste in art. But that’s O.K. too. As Ms. Susman said, “It’s great for getting the conversation going.”