James T. Demetrion, the longest-serving director of the Hirshhorn Museum and Sculpture Garden, an institution he transformed over 17 years from an eclectic and haphazard collection of paintings and sculptures into a world-class center for modern and contemporary art, died Nov. 29 at a hospital in Arlington, Va. He was 90.

The cause was complications from a stroke, said his daughter, Elaine Demetrion.

When Mr. Demetrion became the Hirshhorn’s director in 1984, the Smithsonian museum was perhaps best known for its striking architecture, occupying a doughnut-shaped fortress of granite and concrete on the Mall. It had opened a decade earlier with a sprawling collection donated by financier Joseph Hirshhorn, who acquired thousands of works by artists including Edgar Degas, Randall Davey, Alexander Calder and Auguste Rodin.

But while the Hirshhorn was billed as a contemporary and modern art museum, it had few if any works from recent decades. Its holdings included pre-Columbian artifacts, Persian miniatures and two Picasso paintings that Mr. Demetrion dismissed as “Park Avenue pictures,” worthy of a millionaire’s apartment but not a top-tier museum.

Mr. Demetrion set about honing the collection, pruning it of lesser works and pieces that distracted from its central mission. He shaped what Nicholas Serota, director of the Tate in London, once called “one of the great collections anywhere in Europe or America,” while acquiring a reputation as one of his field’s most beloved and idiosyncratic leaders.

“To say that Jim was a singular figure in the art world, in the museum world, would be an enormous understatement. There was frankly no one like him,” said Neal Benezra, a former chief curator at the Hirshhorn and current director of the San Francisco Museum of Modern Art.

“Here’s a man who’s very grounded, very focused on the art, not seduced by the glamour of the art world, but at the same time was a connoisseur of the first order,” Benezra added in a phone interview. “That was a unique combination.”

Unlike other museum directors, Mr. Demetrion never received a PhD or MBA. He loathed hobnobbing with wealthy benefactors, waxed poetic about Pete Rose and his beloved Cincinnati Reds, never owned a tuxedo until his colleagues at the Des Moines Art Center gave him one as a gift for his new job at the Hirshhorn, and considered hamburgers the finest food ever created, even using “hamburger” as a term of endearment for friends.

“He was absolutely unpretentious,” said artist James Turrell, who recalled meeting Mr. Demetrion as a student at Pomona College in California, where Mr. Demetrion briefly taught art history in the 1960s and was “instrumental” in sparking his interest in art. Long before kale became commonplace, “he once joked that he only ate vegetables with one-syllable names,” Turrell said.

Mr. Demetrion was far more serious about “building culture,” as he put it, by acquiring new works of art for museums. “A museum that begins with a private collection and doesn’t continue to collect becomes a tombstone,” he once told The Washington Post, and to that end, he acquired works by artists including Lucian Freud, Gerhard Richter, Jasper Johns and Claes Oldenburg.

“Jim knew great art, and he had a unique capacity to step up and get great art,” said Robert Lehrman, a Washington art collector and Hirshhorn trustee who chaired the board during Mr. Demetrion’s tenure. In a phone interview, he recalled that while Mr. Demetrion’s acquisition budget was relatively paltry, he cultivated relationships with artists including Ed Ruscha and Clyfford Still that helped him land top works.

Those included a haunting portrait of Pope Innocent X by Francis Bacon that Mr. Demetrion purchased for the Des Moines Art Center earlier in his career. The acquisition was a remarkable coup for a regional art museum, said Lehrman, who described it as the art-world equivalent of Mickey Mantle going to the hapless Washington Senators while at the height of his powers, instead of remaining with the powerhouse New York Yankees.

The current director of the Hirshhorn, Melissa Chiu, said that while Mr. Demetrion was known for acquiring masterworks, he was also “responsible for really looking at diversity within the collection.” Alongside works by White men from Europe and the United States, he added pieces by artists such Nam June Paik, Ana Mendieta, Glenn Ligon, David Hammons, Doris Salcedo and Anish Kapoor.

Mr. Demetrion “built up an acquisition war chest,” said Chiu, growing the Hirshhorn’s annual acquisitions budget from $150,000 when he joined the museum — it was half his budget in Des Moines — to an endowment of more than $30 million. The growth was fueled in part by the sale of works he viewed as inessential, which provided cash to buy more art.

He also organized major retrospectives on Still, Bacon, Jean Dubuffet and Stanley Spencer; renovated the museum’s plaza to better integrate large outdoor sculptures with the landscaping; hired curators who became leaders in the field, including Benezra, Olga Viso and Ned Rifkin, his successor as Hirshhorn director; and avoided major controversies over censorship or offensive works of art.

“His integrity was obvious,” Post art critic Paul Richard wrote in 2001, after Mr. Demetrion retired from the museum. “He wasn’t out to shock, or to stroke the market, or to bow to those in power. He was out to show us art.”

The older of two children, James Thomas Demetrion was born in Middletown, Ohio, near Cincinnati, on July 10, 1930. His parents were Greek immigrants who worked in the restaurant business; his father pushed an ice-cream cart in the summer.

Mr. Demetrion studied education at Miami University in Ohio, where he received a bachelor’s degree in 1952, and became interested in art while stationed in Europe with the Army, visiting museums and marveling at paintings by Peter Paul Rubens. While teaching high school social studies in California, he began studying — and then teaching — art history as well.

“I had the audacity to teach an art course to adults, at night, in a local junior college,” he later recalled. “I was just two pages ahead of the students, but by then I was hooked.”

While doing graduate work at UCLA, he met Walter Hopps, director of the Pasadena Art Museum in California, who organized the first American retrospectives of Marcel Duchamp and Joseph Cornell and groomed Mr. Demetrion as his successor.

Mr. Demetrion led the Pasadena museum for about three years before being named director of the Des Moines Art Center in 1969. He briefly served as interim director of the Menil Collection in Houston at the close of his career.

In 1954, he married Barbara Parrish. In addition to his wife, of Arlington, and daughter, of Austin, survivors include a brother and two grandsons. An annual lecture series at the Hirshhorn was established in his honor in 2001.

Mr. Demetrion said he never saw the Hirshhorn as a competitor to larger institutions such as the Museum of Modern Art in New York City. Instead, he told the New York Times, its smaller size gave it the “opportunity to be broad,” with room “for wit and human foible” in its selection of pieces.

Those included art works that Mr. Demetrion sometimes spent years pursuing, including a piece by sculptor Mark di Suvero that he waited more than two decades to acquire, as well as a mustard-colored painting by Still that he sought out after turning down another work by the abstract expressionist.

The painting filled a void in the museum’s collection, he told The Post in 1997, but also had a broader significance. “It just looks like a slice of the cosmos to me,” he said.

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