Ms. Stern was a multifaceted whirlwind of energy who worked in her studio, chaired the board of a museum and hosted dinner parties for Washington’s political insiders, all while raising five children.
She moved to Washington in 1957, after marrying Philip M. Stern, a journalist, author and heir to the Sears, Roebuck fortune. In the late 1950s, Ms. Stern and two business partners operated a company that rented artwork to businesses and individuals.
She was also a co-founder of the Washington Gallery of Modern Art, which opened in 1962 near Dupont Circle with the first major retrospective dedicated to Franz Kline, a major 20th century artist.
As a member of the museum’s board and, for a time, its acting chairman, Ms. Stern helped establish a place for contemporary art in Washington. She was an early champion of the work of artists in the movement known as the Washington Color School, including Kenneth Noland, Morris Louis, Gene Davis and Paul Reed.
In its six years of existence, the Washington Gallery of Modern Art presented exhibitions of such celebrated figures as Arshile Gorky, Frank Stella, Robert Rauschenberg, Jasper Johns and Edward Kienholz. It also helped propel the careers of several Washington artists, such as sculptor Anne Truitt, painter Sam Gilliam, printmakers Lou Stovall and Di Stovall, and Rockne Krebs, who created environmental sculptures with beams of light.
“Leni was a brilliant artist herself,” Lou Stovall said in an interview, “and she did a lot for other people who were making art. But she never really received the credit she deserved.”
In 1967, as the Gallery of Modern Art faced financial struggles, Ms. Stern led a fundraising and membership drive that restored a measure of stability. That year, she was influential in hiring Walter Hopps as the museum’s final director before it merged with the Corcoran Gallery of Art in 1968. Hopps, who worked at museums from coast to coast, was considered one of the most visionary curators of the 20th century.
Ms. Stern was a pianist and painter before turning to sculpture. She had several gallery shows in the 1970s featuring her Plexiglas sculptures, some which were more than six feet high.
“These are complex works,” Washington Post art critic Paul Richard wrote in 1971. “To see them one must move, for interacting there within those spires are voids and planes, sharp edges and soft Brancusi curves. Seen, from here, one internal plane is just transparent; move, and it becomes a mirror; move again and it refracts and little rainbows shine.”
Her sculptures were later purchased by the Woodruff Foundation, which placed them at embassies around the globe.
Helen Phillips Burroughs was born July 4, 1930, in Manchester, N.H. Her mother was a homemaker, and her father was an insurance executive who designed pension and profit-sharing plans and was an unofficial adviser to President Dwight D. Eisenhower.
Ms. Stern attended Wells College in Aurora, N.Y., and Smith College in Northampton, Mass., before her marriage in 1950 to Henry Sedgwick. They later lived in Montreal before their eventual divorce.
In 1957, while recovering from tuberculosis in Barbados, Ms. Stern met Philip Stern, who was an adviser to two-time Democratic presidential candidate Adlai E. Stevenson. Stern also worked on Capitol Hill, founded the Arlington Sun newspaper in Northern Virginia and was an assistant secretary of state during the administration of President John F. Kennedy.
In 1968, Ms. Stern and her husband co-wrote a book, “O Say Can You See By the Dawn’s Urban Blight,” which used photographs to highlight the contrasts among the District’s wealthier and poorer neighborhoods. They were divorced in 1972.
Ms. Stern participated in antiwar demonstrations and in 1969 helped organize a meeting in Toronto of women from the United States, Canada and North Vietnam.
“What governments will not do, the people must do, perhaps through a separate peace arranged by a supra-government,” she said at the time. “After all, this is an undeclared war.”
In 1976, Ms. Stern received a bachelor’s degree in anthropology from George Washington University. For many years, beginning in the mid-1970s, she maintained a home in Cuernavaca, Mexico. She was an art teacher and benefactor for a Mexican orphanage before returning permanently to Washington in the early 1990s.
Survivors include her partner of 25 years, William W. Upton of Washington; three children from her first marriage, who were later adopted by Philip Stern, Henry Stern of Potomac, Md., jazz guitarist Mike Stern of New York and Holly Sedgwick of Toronto; two children from her second marriage, David Stern of Chevy Chase, Md., and Eve Stern of Tucson; a sister; 11 grandchildren; and seven great-grandchildren.
During their marriage, Leni and Philip Stern were significant art collectors. Their home in Washington’s Kalorama neighborhood was a social hub for journalists and diplomats and also doubled as a de facto museum.
In 1978, Ms. Stern sold much of her art collection, which included works by sculptor David Smith and painters Marsden Hartley, Josef Albers and John Marin. She also had many works by the Washington artists who had risen to prominence in the 1960s after exhibitions at the Washington Gallery of Modern Art.
“As wise and generous and courageous patrons of contemporary art,” Richard wrote in The Post in 1971, “she and her husband, Philip, have no local peers.”