His wife, Marilynn, said he died of natural causes.
Mr. Karp always found the time to look at the slides of dozens of aspiring artists who each week asked him for the chance of a lifetime. He responded with precious gallery space, encouragement, a referral to a more appropriate dealer or rejection.
As Pop blossomed, Mr. Karp took to the road and made hundreds of speeches promoting the new art form not to mention appearances on television shows like "The Tonight Show."
In 1969, Mr. Karp was one of the first gallery owners to follow artists to SoHo, an industrial area that would quickly become as much an art gallery district as the Upper East Side. His O K Harris Works of Art first at 485 West Broadway, then at its present address, 383 West Broadway would become the first gallery on one of SoHo's principal boulevards. He, like several other pioneers to venture south of Houston Street, liked to think of himself as SoHo's unofficial mayor.
As driven as he was to find the new, Mr. Karp also gained fame for foraging through demolished buildings for architectural artifacts and remnants, thousands of which were donated to museums by a society he founded. He and his wife collected all manner of odd things from washboards to misspelled restaurant menus to hatbox papers. A recent passion was to restore old country burial grounds.
But his life's mission was to help find and guide the careers of major 20th century artists, among them Claes Oldenburg, Tom Wesselmann and John Chamberlain in addition to Warhol, Lichtenstein and Rauschenberg. He became an early champion of photorealism, the genre that arose in the late 1960s in which painters strive for a photographic effect.
He worked at the search. The New York Times Magazine in 1966 quoted an unnamed art expert as saying, "He knows as much as, if not more than, anybody else about what's going on in the hidden corners of the art world."
Once he found a potential genius, Mr. Karp's would do everything he could to help the artist achieve rewards commensurate with his talents.
Citing van Gogh's fame, which came only at the end of his life and after his death, Mr. Karp vowed, "No genius should go undiscovered."
He never claimed this commitment was altruistic. In 1968, The Times called him "New York's deftest and most enthusiastic salesman of the new art."
Ivan Karp was born on June 4, 1926, in the Bronx. His father, a hat salesman, soon moved the family to Brooklyn. Mr. Karp dropped out of Erasmus Hall High School to join the Army Air Forces in 1944. He recalled taking the middle name Conrad when he was standing in line to enlist. He noticed that men without middle names were marked "no M.I.," for middle initial and thought it looked dreadful. He was carrying "Heart of Darkness" by Joseph Conrad at the time.
Mr. Karp attended The New School for Social Research on the G. I. Bill and was one of the first arts critics for The Village Voice in the mid-1950s. He also published short stories. At some point he sold Good Humor ice cream and tried to organize a union for the company's workers. At another, he was employed by movie companies in New York to edit romantic scenes out of Westerns in the belief that fans liked only galloping and guns.
In 1958, he became an art dealer at the Martha Jackson Gallery. The next year, he moved to the Leo Castelli Gallery to be associate director. There, the loud, affable Mr. Karp became an excellent counterpoint to Mr. Castelli, a soft-spoken sophisticate who helped win international acceptance of postwar modern art.
Marilynn Gelfman, a sculptor, came into the gallery in 1964 with a friend's paintings that other dealers had found too unorthodox. "He said he liked my friend's work, but he loved me," she said. They married later that year.
In addition to his wife, Mr. Karp is survived by his sons, Ethan and Jesse; his daughter, Amie Karp; his sister, Rhoda Ben-Isaac; and two granddaughters.
In 1967, Mr. Karp published a comic novel, "Doobie Doo," about love between Pop artists. The next year, he yearned to start his own gallery at the "outposts of civilization" and headed for SoHo. He named the enterprise big enough to hold five one-man shows at once O K Harris because it was "a tough, American name that sounded like that of a riverboat gambler."
As president of the Anonymous Arts Recovery Society, Mr. Karp trundled about New York in a beat-up Jeep looking for gargoyles, capitals and cornices from buildings that had been torn down. A particular delight was finding carved portraits that Italian immigrant stone craftsmen had made of one another warts, missing teeth and all.
Mr. Karp, a self-described "rubble-rouser," accomplished his preservation mission the old-fashioned way: with bribes. "For anywhere between $5 to $25, they'll take the trouble not to smash something," he said.