WASHINGTON — Herbert Vogel, a retired New York postal worker who, with his wife, Dorothy, created one of the world’s most unlikely — and most significant — collections of modern art, then bequeathed much of it to the National Gallery of Art, died July 22 at a nursing home in New York City. He was 89.

His death was confirmed by Anabeth Guthrie, a spokeswoman for the National Gallery. The cause of death was not immediately disclosed.

In 1962, when Mr. Vogel and Dorothy Hoffman were married, they came to Washington on their honeymoon and spent several days visiting the National Gallery and other museums. When they returned to New York, they began to buy a few pieces by artists they met, slowly amassing their collection.

Unlike many collectors, the Vogels were not wealthy people. They lived and collected their entire lives on their salaries and their pensions. Mr. Vogel worked nights sorting mail at New York post offices, and his wife was a reference librarian in Brooklyn.

They bargained directly with artists, sometimes buying on installment, paying as little as $10 a month. Once, they received a collage from Christo in exchange for cat-sitting.

The Vogels never talked about how much they paid for a work of art and did not sell a single piece they owned until the National Gallery acquired much of their collection in 1991. Estimates of the collection’s value range into the millions, but National Gallery officials and others who have seen it decline to give a number.

‘‘We could have easily become millionaires,’’ Mr. Vogel told the Associated Press in 1992. ‘‘We could have sold things and lived in Nice and still had some left over. But we weren’t concerned about that aspect.’’

When they began collecting in the early 1960s, the Vogels — known in the art world simply as ‘‘Herb and Dorothy’’ — concentrated on conceptual art and minimalism. It was difficult, edgy work, often with straight lines and little ornamentation, that stood apart from the better-known abstract expressionist and pop art movements.

Their first purchase was ‘‘Crushed Car Piece’’ by John Chamberlain, who made sculpture from wrecked auto parts. It was not the sort of art that was in strong demand.

The Vogels visited studios and became close friends with many artists, including Sol LeWitt, Richard Tuttle, and the husband-and-wife duo of Christo and Jeanne-Claude. They were often the first collectors to open their wallets to buy from unknown artists. Over a period of almost 50 years, the Vogels amassed more than 5,000 works of art, including drawings, paintings, sculptures and pieces that defied classification.

Herb and Dorothy Vogel had three requirements in purchasing art: it had to be inexpensive; it had to be small enough to be carried on the subway or in a taxi; and it had to fit inside their one-bedroom apartment. Over time, they became fixtures in the New York art world. They haunted the city’s galleries and studios, attending as many as 25 art events a week. They studied art magazines and kept in close touch with artists.

What began on a whim built on small purchases grew into a deep and wide-ranging collection that included many of the leading artists of the past 50 years: Chuck Close, Donald Judd, Robert Mangold, Dan Flavin, Joseph Beuys, Brice Marden, Nam June Paik, Edda Renouf, Edward Ruscha, Robert Ryman, Julian Schnabel, Robert Smithson, Carl Andre, Lynda Benglis, John Baldessari, and Jeff Koons.

The Vogels acted on intuition and personal taste, trusting their instincts rather than the advice of high-priced consultants or galleries. Pat Steir, whose paintings often resemble waterfalls, met the Vogels through LeWitt, an artist noted for his geometric paintings and sculptures.

‘‘When they first bought from me, I called Sol and said, ‘What should I charge them?’ ” Steir told W magazine in 2008. ‘‘And he said, ‘Take off three zeros and cut the price in half.’ And then they paid month by month on the installment plan.’’

Artists considered it a privilege to be included in the Vogel Collection and a greater honor to be invited to the couple’s cramped apartment for a meal. Dorothy Vogel would sometimes offer a TV dinner that she warmed up in the oven.

Their small apartment was quickly overrun with art, which hung on the walls and was stacked on the floor and under the bed. They got rid of their sofa and had only enough room to sleep, eat and care for their cats — as many as eight at a time — and the exotic turtles and fish that Mr. Vogel kept in aquariums.

When Mr. Vogel retired from the Postal Service in 1979, he used his pension to buy more art. He and Dorothy began to think about the legacy they wanted to leave, and many top museums came calling.

On their 25th wedding anniversary in 1987, the Vogels paid a return visit to the National Gallery, where their love affair with art — and with each other — had blossomed.

After years of negotiations, the Vogels agreed to send the heart of their collection to the National Gallery. The terms have never been disclosed, but the deal included both purchases (on the part of the National Gallery) and gifts (from the Vogels). ‘‘We wanted to do something for the nation,’’ Mr. Vogel told the Houston Chronicle in 1992. ‘‘The National Gallery doesn’t sell works they acquire. They’ll keep the collection together. And they don’t charge admission.’’

The National Gallery’s Vogel Collection now contains more than 900 works, and almost 300 more have been promised to the museum.