Try to imagine art history without Paul Cézanne, and you have a sense of the influence of Ambroise Vollard — the dealer who brought Cézanne to the attention of Picasso, Renoir, Matisse, and a generation of collectors. Vollard is the subject of an exhibition at the Metropolitan Museum of Art that opens next week and which, in a tacit acknowledgment that the dealer may not be as well-known as his clients, is called “Cézanne to Picasso: Ambroise Vollard, Patron of the Avant-Garde.”

Vollard got his start as a dealer through his keen eye and his chutzpah. Born on the French island colony of Réunion, in the Indian Ocean just east of Madagascar, Vollard came to Paris to go to law school, but quickly tired of it. He got a taste of his future career buying prints from the dealers along the Seine and flipping them for twice the price. He took a post as an intern at another gallery, then opened his own shop on the Rue Laffitte in 1893.

His importance to Cézanne’s career and legacy is immeasurable. When Vollard put on a Cézanne exhibition in his gallery in 1895, the artist hadn’t had a major exhibition in Paris in almost 20 years. Vollard tracked him down through his son and gathered around 150 paintings. “It was a huge sensation,” a curator of the Met show, Rebecca Rabinow, said. “Almost overnight, Cézanne became thought of as a master. All these young artists were flocking to the gallery.” The Met show includes a wall of Cézanne “Bathers” that were purchased by other artists: Renoir, Matisse, Monet, and Pissarro.

Without Vollard’s rediscovering and championing Cézanne, Ms. Rabinow said, the entire history of modern art would have been different. “If he hadn’t chased down Cézanne in 1895, then maybe Picasso wouldn’t have seen it,” she said. “And if Picasso never saw Cézanne, would he ever have gone to Cubism? Because he always said it was Cézanne’s example that led him on that path.”

Vollard promoted other artists, like van Gogh and Gauguin, whose worth dealers and collectors didn’t yet appreciate. He favored figurative, very colorful work. He loved the Fauves — Matisse, André Dérain, and Maurice de Vlaminck. He published numerous books with illustrations by his clients, some of them quite scandalous at the time.

The French government allowed the curators of the Met show access to Vollard’s archives, which his heirs recently transferred to the state in lieu of paying estate taxes. Ms. Rabinow said that she and her fellow curators made several discoveries, among them that Picasso’s “Suite Vollard,” a set of 100 engravings commissioned by the dealer, was initially intended to illustrate a two-volume book.

His gallery was a meeting-place for artists. Unsurprisingly, many artists painted his portrait; Picasso once quipped that Vollard was painted more often than the world’s most beautiful woman. Several of these paintings, including Picasso’s Cubist portrait from 1910, will be in the Met show.

Like many later dealers, Vollard was often difficult to work with; this was as much business strategy as it was eccentricity. He infamously refused to show collectors the paintings they wanted to see. “You read collector’s memoirs,” Ms. Rabinow said, “and they talk about going to Vollard’s gallery. They would ask to see a Cézanne and he would say, ‘Sorry I don’t have one today, come back next week.’ Meanwhile they could see, peeking around the corner, 20 of them! It became notorious that if you wanted an artist you had to ask for a different one.” She added: “It drove up the price, because by the end people were dying to get what they wanted!”

With his eye, and his faith in the artists he loved, Vollard inspired a whole cultural role for the dealer, the writer Adam Gopnik, who has written for the New Yorker on modern art and Parisian culture, said. “The idea that the dealer wasn’t simply a kind of intermediary between artists and collectors, but was a powerful and passionate advocate of a certain kind of art — it would be fair to say that that begins with Vollard.” He was the model for later dealers like Betty Parsons and Leo Castelli.

“His role was very much like that of Sylvia Beach in modern literature,” Mr. Gopnik said, referring to the founder of the Paris bookstore Shakespeare and Company. “She was the one person to have the courage to publish ‘Ulysses,’ and he was the one person who had the courage to buy and sell Cézanne when nobody else thought it was of any value.”