NEW YORK -- When a diamond-shaped Mondrian painting sold for a shade over $5 million at auction last November, the art world was stunned, but the art dealer was not.

Sidney Janis had been on Mondrian's trail since 1930, when he saw a small geometric composition in a Paris exhibition. Janis, at the time a successful shirt manufacturer, was also a budding art connoisseur, snapping up Picassos, Le'gers, de Chiricos, Dalis and Klees. On his next Paris trip in 1932, he paid Mondrian a visit in his studio and bought a painting for the equivalent of $70.

Diminutively robust at age 91, Sidney Janis recently sat in his book-lined private office, adorned with art masterpieces, six floors above the rumbling traffic of West 57th Street, and reminisced.

"I thought I could take it with me after I paid for it, but Mondrian said no, he had to give a little area at the bottom -- about 1 by 4 inches -- another coat of blue. He always had trouble with blue anyway," Janis confided matter-of-factly. "At any rate, I didn't get the picture for almost a year and when I got it, Mondrian had framed it and included a nice little note about the painting."

Asked about the note, Janis kept his eyes focused on the elongated Giacometti sculpture nearby, "Walking Man in the Sun," and said softly, "Well, I'm publishing that," and after another moment volunteered, "Mondrian expressed some feelings about being attached to the picture, but he was glad that it had fallen into my hands."

Sidney Janis is still in the thick of the art world, having not long ago donated more than 100 works from his collection to the Museum of Modern Art. And he continues to buy, sometimes extravagantly. In May, he paid $2.53 million at auction for Giacometti's "Grande Femme Debout III" and said at the time that though he would eventually have to sell it, "I still have a collector's instincts. I can't afford to hold on to it forever, but personally I like it very much."

On the windowsill, a large stationery box holds his nearly completed manuscript about his life and times. Sculptures by Giacometti, Arp and Brancusi punctuate the room like an elite guard.

On the wall next to the curvy Arp bronze are three awards: one from the French minister of culture honoring his contribution to French art; a second, more recent one from the City of New York proclaiming: "For over half a century you have been this city's chief astronomer for the visual arts"; and a third, an indecipherable honor plastered with official-looking seals and a slightly askew blue and red ribbon. The fancy calligraphy cloaked the scribe, the great satirist and object maker Saul Steinberg, who exhibited for years at the Janis gallery. So this third honor was a well-placed spoof on Janis' still-extraordinary career.

Over the years, going back to 1926, when he and his late wife Harriet (better known as "Hansi") purchased a Whistler etching, thousands of paintings and sculptures have passed into and out of his hands. More than 100 examples spotted by his legendary eye are owned by the Museum of Modern Art, where a roomful of Mondrians hang in his honor. As part of his gift, Janis donated seven major Mondrian oils and one watercolor. And it is his friendship with Mondrian that forms a large part of his reminiscences.

That friendship intensified after the artist immigrated to New York in 1940, joining a growing refugee colony of Europe's most avant-garde artists. Those artists shared a passion for ballroom dancing and the new boogie-woogie music. Mondrian only had two solo exhibitions during his lifetime, both at the Valentine Dudensing Gallery in New York, and as Janis recalled, only one picture was sold.

Mondrian died in 1944, four years before Janis opened his own gallery, but Janis remained a tireless advocate of Mondrian's work and controlled the market for his paintings to the meager extent it existed. "It didn't do us a lot of good," Janis said. "We sold about one picture a year."

But that first Mondrian Janis purchased, "Composition," became part of a unique portrait commission by the sculptor George Segal at the time Janis announced his gift to the Modern. It rests on the English easel off which Janis sold countless pictures in his showroom. The 5-foot-5, life-size dealer standing next to the easel is cast in Segal's signature white plaster skin. The bow-tied aficionado admires the geometry of the Mondrian, one hand touching the white frame in the effortless pose of a true connoisseur.

Segal vividly recalls Janis' visit to his New Jersey studio in 1967 for the life cast: "It was a hard job. I was putting plaster over his sports jacket, and in my nervousness I was trying to go very fast. I left out a seam and the plaster dried rock hard. I was sweating and grunting, using powerful tin snips to try and cut the plaster to get it off his jacket. All this sweat was pouring down my forehead and I was about three inches away from him. He was grinning at me and said that Ce'zanne had his dealer, Ambroise Vollard, pose for his portrait 64 times in 1899 and at the end of it, Ce'zanne told Vollard, 'I think I like the shirt front.' Sidney told me this as I was struggling to get him out of the plaster.

"I know that Sidney's great private passion was Mondrian's work," continued Segal. "I thought it was fitting to make a portrait of him with his favorite painting. So when he went so far as to donate his favorite easel I grinned and said, 'Will you put the original Mondrian in?' Without hesitating he said, 'But of course.' Segal called the exchange mind-boggling. "I could have painted a Mondrian {facsimile}. That's what I was thinking of doing."

Janis also commissioned two other pop favorites, Marisol and Andy Warhol, to create portraits commemorating the MOMA gift. Marisol's literally stands out. Aptly titled "Portrait of Sidney Janis Selling a Portrait of Sidney Janis," the larger-than-life-size sculpture portrays Janis in his jaunty bow tie at the epiphanic moment of the sales pitch. He is selling himself. The dealer becomes a work of art.

Marisol appropriated the pose from a photo of Janis and his two sons (his younger, Carroll, 55, runs the gallery today) surrounded by masterpieces of Le'ger, Brancusi and Arp. In the photo, Janis has his right foot perched -- in proprietary fashion -- on the base of the Arp. Marisol kept that pose but substituted a cross-armed rendition of Janis for the abstract Arp. The salesman and dealer stand toe to toe in chunky wooden relief, as serious as Rembrandt's turbaned gentleman contemplating the bust of Homer.

Both Segal and Marisol have had long associations with Janis, and despite their divergent styles, capture the multiple essences of this businessman-connoisseur. Janis meandered considerably before taking up the cudgel for modern art. A high school dropout, he got his start in vaudeville, dancing at the Palais de Danse in his home town of Buffalo. It was still a long way to Paris.

Today at 91, Janis still tangos and mambos on Friday nights with a group of nimble-footed seniors at a professional dance studio in Manhattan. (His old favorite, Roseland, has fallen on hard times.) Still a showman, he wowed the crowd at his 90th-birthday bash at MOMA with a brief tango demonstration.

Janis made his early fortune in shirts, opening the M'Lord Shirt Co. in New York in 1926, with his bride acting as bookkeeper. The couple collaborated in both art and business and after 13 years of selling the novel two-breast-pocket white shirt that followed the leisure set to Florida, Janis closed the business and became a full-time art writer. "I could have done just as well in business if I had remained," Janis said, "but I felt I had something on my mind that I had to get down on paper."

In less than a decade, Janis wrote three books on art, spanning his ever-widening taste to encompass modern masters like Picasso (he spent nine weeks camped out in Picasso's Paris studio doing research), the still-unknown American avant-garde painters like Jackson Pollock and Willem de Kooning and the even farther out American primitives, including Morris Hirshfield, also a retired clothing manufacturer.

Janis not only made money, collected art and wrote; he was also an influential member of the Museum of Modern Art's Advisory Committee, which bristled with Rockefellers and brilliant art historians such as Meyer Shapiro and architect Philip Johnson. He organized a number of key exhibits for the museum, including a much maligned one-man show in 1943 of Hirshfield paintings that one New York critic attacked, describing Janis' exhaustive analysis as "a combination of preciosity and the hunting down of butterflies with the aid of caterpillar tractors." Such criticism of Janis' taste is rare.

When the Sidney Janis Gallery opened on 57th Street in September 1948 with a Fernand Le'ger exhibit, a small article in Art Digest quoted the gallery's aims: " ... the 57th Street visitor can form a standard of appreciation based not only on proven esthetic values in modern art, but also on his own recognition of such values in the work of new painters. To further this serious appraisal and to afford collectors an opportunity of participating in the discovery of new talent through such direct comparison, will be major objectives of the gallery."

The rest, as the saying goes, is history. Janis went on to promote and make waves for the entire stable of abstract expressionists from Jackson Pollock to Mark Rothko. He was not the first to show an interest in their careers -- other dealers like Peggy Guggenheim and Betty Parsons were first -- but he got his ever-growing clientele on the trail of the greatest American art movement.

William S. Rubin, director of the Department of Painting and Sculpture at MOMA, says, "Sidney took them on -- American art had been treated as an also-ran . . . and Sidney insisted on handling, talking about and showing Pollock and de Kooning as if they were on the level of Picasso and Mondrian." (It was Rubin who approached Janis and asked him what plans he had for his collection. Rubin first met Janis as a college student and would often go to the gallery and talk art with the dealer.)

It was not just the pictures that Janis gave to the museum that makes his contribution so great, but also the gifts made by other collectors who originally bought their pictures from Janis. Nelson Rockefeller's gift of Henri Rousseau's "The Dream" was first acquired by Janis in 1934. The picture once belonged to Ambroise Vollard. Rubin calls it "one of the important icons of the Museum of Modern Art's collection."

The first picture one sees upon entering MOMA's Nelson A. Rockefeller Gallery is the Janis Picasso "Artist and Model" from 1928, another of the icons Rubin referred to. Major paintings and sculptures from the Janis Collection anchor the museum's permanent collection galleries, from the tremendous futurist painting "Dynamism of a Soccer Player" by Umberto Boccioni from 1913 to Jackson Pollock's "One" from 1950, a 17-foot-long painting that Janis once sold for $8,000 only to buy it back in 1968 for $350,000 so he could give it to the museum. MOMA has been mentor and muse for Janis ever since it opened in 1929. The heft of his gift underscores the effectiveness of the world's premier modern art muse.

Janis has virtually covered -- through collecting, exhibiting, promoting and selling -- the spectrum of 20th-century European and American art movements, from cubism to futurism, surrealism to abstract expressionism, pop, and his latest love affair, graffiti, or what he likes to call "street art."

As one flips through the museum-quality catalogues published by the gallery (one, "Five Years of Janis," is tattered and dog-eared, while the 25th-anniversary catalogue from 1973 is still shiny), the breadth and depth of Janis' commitment to modern art jumps from the pages, from his first show of Mondrian in 1949 to a double homage to Brancusi and Giacometti last year.

He has shown the modern masters in often eccentric combinations. A gallery is chock full of Mondrians hung at varying heights, steep as a stairway. A mirror is strategically placed in the middle of the floor to give another view of a de Chirico. An unheard-of cluster of Giacometti sculptures and Dubuffet paintings merge in a crowded corner to rattle the gallery like a sonic boom.

Shifting his gaze and the position of his olive leather chair to the polished bronze sheen of Brancusi's "Fish" from 1926, Janis took it in with a satisfied chuckle. "These are the things the gallery is stuck with. At least half of the pictures in my collection at the Museum of Modern Art are paintings that the gallery unsuccessfully tried to sell. And after a few unsuccesses, I decided to keep them for myself. In other words, if our gallery had been more successful, the gift to the Modern would have been less great."

Shifting again in time to his first Mondrian, Janis said, "I remember one time someone came to our house and offered $70,000 for the Mondrian. It was a hell of a jump from $70. But we didn't accept it. Today at auction the same picture would bring a million or more. But the whole thing is academic."

Looking back at the mercurial turns of America's foremost art collector-dealer, it appears Mondrian was right when he said his picture had fallen into the right hands.