Peter Arnell, the New York branding and design impresario, has had a secret preoccupation. For decades, he has been taking photographs everywhere, with anything from an Instamatic camera to a Leica to, most recently, his iPhone. “That’s how I find my ideas,” he said. “All my thinking comes out in my photographs.”

In the 1980s, he tried his hand at fashion photography, shooting Paris couturiers like Christian Lacroix and Emanuel Ungaro. Many of his pictures were published in Vanity Fair, British Vogue and The New York Times Magazine. He also created a celebrated image on a DKNY advertisement.

But in recent years, Mr. Arnell has been a more private shutterbug, using his iPhone, mostly, to chronicle his travels. He has pictures of the streets of Tokyo and Bangkok, Rome and Paris, and New York. And except for some of these images he has given to friends for Christmas, almost no one has seen the thousands of snapshots in his files.

No one, that is, except Frank Gehry. They have been working together — Mr. Arnell even wrote his first book about that architect — since 1979, when Mr. Arnell showed up at Mr. Gehry’s Venice Beach, Calif., studio. “I’ve been encouraging Peter to show his photographs for years,” Mr. Gehry said in a telephone interview, adding that he sees them as fine-art photography, some even “reminiscent of those great black-and-white Seurat drawings.”

Now Mr. Arnell’s work will be publicly exhibited for the first time in a show at the Milk Gallery, 450 West 15th Street, in Chelsea, from March 4 through April 1. Mr. Gehry has agreed to design the exhibition.

More than 100 black-and-white photos will be on view, with portraits of celebrities like Andy Warhol and Christy Turlington. There will also be street photography, architectural images and still lifes.