William Lieberman, who died Tuesday at age 81, was chairman of 20th-century art at the Metropolitan Museum of Art and a “collector of collectors,” as renowned for drawing important gifts and bequests as for his curatorial eye and ability to mount a blockbuster show.

Lieberman enjoyed a 60-year career in art museums spanning two New York institutions: the Museum of Modern Art, which he joined in 1945 as an assistant to its founding director, Alfred Barr; and the Met, which he joined in 1979 as curator of its 20th-century collections.

At MoMA, Lieberman established the department of prints in 1949 and the department of drawings in 1971. He served as director of painting and sculpture until his departure, in 1979. Lieberman organized more than 40 exhibitions and oversaw a number of major acquisitions, including Gertrude Stein’s collection of Picasso and Juan Gris works. Regarding himself as the natural successor to Barr, who retired in 1967, Lieberman left after a protracted leadership dispute with MoMA’s eventual director, William Rubin.

Although while at MoMA he had been known to say that the Met should stick to old art, once he came to work at the Met, Lieberman threw himself into building its modern collections with a zeal that threatened to make the museum truly encyclopedic. He oversaw the construction of the Lila Acheson Wallace wing, which opened in 1987 to house the museum’s 20th-century holdings. He also husbanded several of the museum’s most spectacular acquisitions, including the collection of Natasha and Jacques Gelman in 1998, estimated at the time to be worth $300 million, and that of Florene May Schoenborn in 1996, estimated at $150 million. Other donations – both to MoMA and the Met – were solicited directly from artists or their representatives, including Henri Matisse, Paul Klee, and Jackson Pollack, an artist whose work Lieberman helped to authenticate in several notorious cases. So assiduous was Lieberman in courting donations that it was rumored he had purchased haberdashery for some benefactors and even walked their dogs.

If one wanted to design a curator of modern art, one could do worse than to have him born in Paris on Valentine’s Day of 1924, as Lieberman was, to parents who were friends of such art-world figures as Picasso, Matisse, and Stein. Brancusi once smacked him for touching a polished sculpture.

His father was a medievalist who delighted in archival research, and Lieberman got his early experience as an “object man” by spending hours at the Louvre and the British Museum. He conducted impromptu tours for pocket money. His only memory of Stein before World War II was of Alice B. Toklas giving him candy. The family moved back to New York in the mid-1930s – his father had grown up on the Lower East Side and had an independent income that allowed the family to move at will – and Lieberman attended Townsend Harris High School. After attending Swarthmore College, he began volunteering at MoMA. His college tutor, W.H. Auden, recommended him to Paul Sachs, who conducted a well-reputed museum course at Harvard University, which Lieberman attended in 1944-45. He then joined MoMA as Barr’s assistant, and within four years he was the museum’s first curator of prints, appointed when the museum opened the Abby Aldrich Rockefeller Print Room.

Among the exhibits he curated at MoMA were “Modern Masters: Manet to Matisse,” Max Ernst, Joan Miro, Modigliani, Pollock, and “Art of the Twenties.” While the Gertrude Stein collection represented the apogee of his accomplishments for acquiring art for MoMA, he also managed to arrange for major gifts by a number of artists, including Matisse and Jean Debuffet.

When Lieberman went to the Met, his arrival was greeted by expectations that the museum would finally begin to address art of the 20th century with the kind of rigor and enthusiasm the institution had for objects from Egypt, say, and medieval and Renaissance painting. In interviews, Lieberman insisted that the Met had to install modern art into its encyclopedic vision in order to stay relevant to its donors. “Ninety-nine percent of all collectors – the rich, those who are interested and will support museums in the future – are collectors of contemporary art,” he said in 1985. “We are not going to cut ourselves off from the collectors of the future.”

The Met’s chairman, Philippe de Montebello, admired Lieberman’s forthright manner as well as his winning ways with benefactors. “He lectures me. He lectures everybody. I’ve heard him lecture donors,” Mr. Montebello was quoted as saying in a 1987 New York magazine article. The donors themselves seemed to find his attentions less than abrasive. A longtime trustee-chairman of the drawings department, Lily Auchincloss, recalled his coming by on Sunday nights to watch episodes of “Upstairs Downstairs.” Lieberman’s personal relationships with donors were certainly a major factor in his success in attracting their gifts and bequests.

Oddly, Lieberman’s first Met show was 50 paintings on loan from MoMA, while that museum put on a blockbuster Picasso exhibition. Major retrospectives of Henry Moore and Balthus followed.

The opening of the Wallace wing in 1987 generated 60,000 square feet of gallery space (more than 50% of the total at MoMA at the time) for modern art. Lieberman’s shows in the new space illustrated an ecumenical attention to masters both past and living, including David Hockney, Boccioni, Rene Magritte, Lucian Freud, Dali, and R.B. Kitaj.

As he had hoped, the Met’s dedication to modern work quickened the pace of major donations, with the Gelman and Schoenborn bequests leading the way. In 2003, 154 major works from the Pierre and Maria-Gaetana Matisse Foundation were donated. They became the basis for Lieberman’s final show, a year-long exhibition of Matisse that folds at the end of this month.

In 2004, Lieberman accepted the title of Jacques and Natasha Gelman chairman emeritus of the newly reorganized 19th-century, modern, and contemporary-art department. Although slowed in recent months by heart problems, he was at work daily as recently as last week, and his death was not expected.

William S. Lieberman

Born February 14, 1924, in Paris; died May 31 at home in Manhattan of cardiac arrest; he never married.