A butter yellow house that stretches for most of the block doesn’t conceal itself from the world. But this Georgetown mansion is a house of secrets — secret doors, secret passageways — and if the walls could talk, they could probably give up plenty of secrets from the Washington power brokers who met here.

Its best secret, though, is the garden behind the house.

“The garden was always very, very beautiful,” said C. Boyden Gray, a former ambassador to the European Union whose parents bought the house in 1957.

The circa 1840 dwelling began as two houses, which were merged around 1929 by Ruth Hanna McCormick Simms and transformed into the stately home that exists today. Simms was the daughter of one U.S. senator, Mark Hanna, and the wife of another, Medill McCormick. She bought the houses not long after she was elected to Congress.

Distinguished homes for sale in the D.C. region

Georgetown mansion | The circa 1840 mansion began as two houses, which were joined in 1929. It is listed at just under $9 million. (HomeVisit)

Simms hired Rose Greely, the first female architect licensed in the District, to design the garden. The garden was featured in House Beautiful in 1933 and has been a regular stop on Georgetown garden tours.

Perry Wheeler, who helped design the Rose Garden and Washington National Cathedral’s Bishop’s Garden, and Andrea Filippone of F2 Environmental Design, a boxwood expert, have updated and revitalized the garden over the years.

In a 2014 Washington Post story, gardening columnist Adrian Higgins called the garden “a masterpiece of spatial arrangement in a contained urban environment.”

The property also has two greenhouses, one of which Gordon Gray, Boyden Gray’s father, used to grow orchids.

“He became quite a fairly important expert in breeding [orchids],” Gray said.

After Simms, a succession of boldface names lived in the home: William A.M. Burden, a New York financier who was assistant secretary of commerce and later ambassador to Belgium; John Balfour, who leased the home during his tenure as a high-level British diplomat here; and Scottie Fitzgerald, the daughter of F. Scott Fitzgerald.

Gordon Gray and his wife, Nancy, came to Washington after President Dwight D. Eisenhower appointed him to head the Office of Defense Mobilization. He later became Eisenhower’s national security adviser. He served on the President’s Foreign Intelligence Advisory Board in four administrations.

Boyden Gray says his parents selected the 10-bedroom house mainly to accommodate their blended family. Gordon had four sons and Nancy had three daughters from previous marriages.

“You needed space to have seven running around,” Gray said.

The house has all the elegant Old World touches expected in a stately mansion — Adamesque marble mantels in the living room, dentil and Greek key molding in the library and a wood floor laid in an exquisite pattern in the dining room. There’s an oval breakfast room and a sunny conservatory. It was featured in Architectural Digest in 1994.

But the unexpected passageways are what make this house special. A concealed door in the library opens to a secret, concrete spiral staircase that would be at home in a European castle. A secret door in a closet of the master suite dressing room connects to an office.

Although his parents hosted many A-list dinners, Boyden Gray fondly recalls pizza parties in the pavilion behind the house. “It was really a glorious place to grow up, especially if you had six brothers and stepsisters,” he said.

The 10-bedroom, seven-bathroom, 11,100-square-foot house, on 0.68 acre, is listed at just under $9 million.

Listing agents: Jamie Peva, Ben Roth and Lucy Blundon, Washington Fine Properties