"A View of Mount Vernon with the Washington Family" by Benjamin Latrobe. It was painted after Latrobe spent the night at Mount Vernon with George and Martha in 1796. Mount Vernon received the painting at auction, with financial help from an anonymous donor. (Courtesy of Mount Vernon)

It is a pleasant evening in July. The dog is running in the yard. And the Washingtons are entertaining on the piazza of their lovely home on the Potomac River.

A young guest is peering through a spyglass toward the water. There’s an urn on a table. And the setting sun is casting shadows on the lawn.

It’s 1796, and President George Washington and his wife, Martha, are portrayed, as they were, in a moment of tranquility.

It is also a scene of such importance that Mount Vernon says it saved one of its treasures when, with the help of a donor, it bought the watercolor at auction last month for $602,500.

Mount Vernon has had the painting on loan since 2004 but faced losing it when it was sent to the auction block in New York, officials said.

“As a loan it was always subject to recall by the owner,” Mount Vernon curator Susan Schoelwer said in an e-mail. “We were heartbroken at the idea that this important item might go elsewhere.”

The painting is a snapshot, although from a distance, of Washington at rest — different from the tight-lipped figure on the dollar bill or the stern, black-clad image in formal portraits.

He’s wearing a blue coat, tan knee breeches and white stockings, and he sits in an armchair beside his wife and her teenage granddaughter. The lawn slopes away, as it does today, to the sunlit Potomac in the background.

The painting, by Benjamin Henry Latrobe, is believed to be, in addition to its intimacy, the only known lifetime image of the first first couple on their piazza, Mount Vernon officials said.

The painting is 1 foot, 51 / 2 inches high by 2 feet, 1 inch wide and is signed on the front, “Benjamin Henry Latrobe.”

Mount Vernon officials said the painting had been loaned to them by Louise Tucker Mentzer. Citing privacy, they declined to identify the donor who helped purchase the painting, say how much the donor chipped in or provide details of the arrangement.

The 217-year-old painting was previously displayed at Mount Vernon from October 2006 to January 2007 and from October 2007 to January 2008.

Susceptible to light damage, it has been in storage in the meantime, officials said. It will go back on display next year.

The image, “A View of Mount Vernon with the Washington Family,” was painted by Latrobe after he spent the night of July 16, 1796, at Mount Vernon, the president’s mansion 16 miles south of the District.

Washington, 64, was taking a summer break from the capital in Philadelphia. He was finishing out his second term as the nation’s first president.

Latrobe, 32, was a British architect who had just moved to America and would later design important parts of the U.S. Capitol and other buildings.

Latrobe found Mount Vernon “of no very striking appearance,” he wrote later, but the view from the piazza was one where “nature has lavished magnificence.”

“The mighty Potomac runs close under (the) bank, the elevation of which must be perhaps two hundred and fifty feet,” he wrote.

As described in his journal, he portrayed the president “in a plain blue coat, his hair dressed and powdered.” Martha Washington sits at the table with the urn.

Latrobe, a widower, spent much of his skill, and words, capturing Martha’s granddaughter by a previous marriage, Nelly Custis, 17, who lived with the Washingtons.

Nelly “had more perfection of form, of expression, of color, of softness, and of firmness of mind, than I have ever seen before or conceived consistent with mortality,” he wrote.

He drew her in a white, floor-length Empire-style dress with a band in her hair. She is leaning against one of the piazza’s pillars. He added her white and brown spaniel, Frish, to the scene — depicted in mid-leap.

There’s also a child in the picture who may be one of the children of a Washington aide, Tobias Lear.

The scene is framed by locust trees in the foreground to one side of the mansion, Schoelwer said in a telephone interview, and two trees in the background that may be white oaks.

Latrobe recounts coffee being served on the piazza at 6 p.m. and he and Washington going inside as it grew dark.

“You can imagine the sun setting on the Potomac,” Schoelwer said. “Conversation . . . continued on the piazza until dark. Then they went inside. You can imagine the mosquitoes.”

Meanwhile, one other figure appears in the painting — the man with the spyglass.

Schoelwer said she’s not sure who he is. He is wearing tight-fitting fashionable trousers, not knee breeches.

It may be Latrobe. Young, stylish, fresh from Europe, he might have worn the latest fashions. Plus, he’s just across the pillar from the striking Nelly.

“Latrobe placing himself there makes the best story,” Schoelwer said. “There is not conclusive evidence . . . but that would be the romantic story you can certainly construct.”