John Carl Warnecke, a San Francisco-based architect whose friendship with the Kennedy family led to some of his best-known designs, including Washington’s historic Lafayette Square and the Kennedy gravesite at Arlington National Cemetery, died April 17 at his family ranch in California’s Sonoma County. He was 91 and had pancreatic cancer.

In a career spanning more than 45 years, Warnecke made a name for himself as an architect whose modernist approach was tempered by a sensitivity for history and the environment. He built one of the country’s largest architectural concerns, John Carl Warnecke and Associates, which took on projects as diverse as commercial skyscrapers, airports, libraries, civic complexes and shopping centers.

Working first with his architect father and later alone, he rose to prominence with well-regarded designs for schools and university buildings in his native California. In the mid-1950s, he won a commission to build the U.S. Embassy in Thailand. His design, described in The New York Times as “a kind of floating pagoda rising up on slender white stilts,” received widespread acclaim.

Kennedy projects

But it was a trip to Washington in the early 1960s that launched Warnecke into the orbit of the Kennedy family. In town to judge a design contest, he accompanied an old friend — Paul Fay, John F. Kennedy’s undersecretary of the Navy — to the White House.

Kennedy, who had studied at Stanford University before World War II, recognized the 6-foot-3 Warnecke as a onetime Stanford football hero. When Kennedy discovered that Warnecke was an architect, he asked him to help with a problem.

The General Services Administration planned to raze the historic townhouses lining Lafayette Square just north of the White House, replacing them with behemoth federal office buildings. Critics argued that the changes would destroy the character of square, originally called President’s Park, and Mrs. Kennedy was sympathetic.

Warnecke proposed to renovate the rowhouses and build office buildings behind them, maintaining the square’s sense of the past. The plan was ultimately hailed as an elegant solution to the problem of historic preservation in an age of rapid urban renewal.

Warnecke was appointed to the Fine Arts Commission by Kennedy and grew so close to the president and his wife that after Kennedy was killed in 1963, Jacqueline Kennedy chose Warnecke to design her husband’s gravesite. His simple plan for a prominent green slope at Arlington National Cemetery was universally praised as an exercise in restraint.

The circular walkway leading to an overlook and white gravestones set into the earth, flanking an eternal flame, was “a statement that was an understatement,” wrote a Times critic.

Warnecke went on to build many other projects domestically and abroad, including the Oakland, Calif., airport, a 40-story hotel in San Francisco and the Hawaii statehouse, a stone building rising from a reflecting pool.

In the Washington area, his notable buildings include several at the Naval Academy in Annapolis, a D.C. shopping center on Wisconsin Avenue and Georgetown University’s brutalist Lauinger Library.

In the 1980s, he built the Hart Senate office building and designed a project in D.C. that, like his Lafayette Square plan, featured a modern office building behind historic rowhouses. Neither project received the glowing reviews of his earlier work.

He retired in the late 1980s to his family’s 80-acre ranch along the Russian River. He grew grapes for wineries and worked on a memoir, which he finished shortly before his death.

Personal life

John Carl Warnecke was born Feb. 24, 1919, in Oakland and received a bachelor’s degree at Stanford in 1941. He graduated from Harvard University’s school of architecture in 1942 and soon joined his father’s architecture firm in the Bay Area. Several years later, he went into business for himself.

In his yet-to-be-published memoir, “The Right Place: Life, Love and Architecture,” he wrote of his affair with Jacqueline Kennedy, with whom he’d fallen in love while the two were working closely to plan the Kennedy family gravesite in the mid-1960s. They saw each other for more than a year, he wrote.

The affair ended when Mrs. Kennedy began seeing her future husband, the Greek shipping magnate Aristotle Onassis. Warnecke remained close to her in-laws, however, and built Robert F. Kennedy’s pool house at Hickory Hill in McLean, Va., and Edward Kennedy’s homes in Hyannis Port, Mass., and McLean.

Warnecke’s marriages to Grace Cushing and Grace Kennan ended in divorce. A son from his first marriage, John C. Warnecke Jr., died in 2003.

Survivors include three children from his first marriage, Margo Warnecke Merck, of Healdsburg, Calif., Fred Warnecke, of San Rafael, Calif., Rodger Warnecke, of Santa Rosa, Calif.; a sister; and four grandchildren.

Long after leaving Washington, Warnecke remained interested in the city’s public spaces, becoming what former Washington Post architecture critic Benjamin Forgey called “a one-man lobbyist for making Lafayette Square a more vibrant place.”

“Our aim should be to enrich the core of Washington,” Warnecke wrote in The Post in 1998. “It should be to bring more life to a city that has become too cold, too monumental and too sterile.”