The photographer, who kept his age and other personal details closely guarded secrets, died Nov. 25, The New York Times reported Friday.
Doctors diagnosed Regan with cancer several years ago, but he only shared the news with a select number of close friends and family in his final weeks, longtime friend Suzanne Guard told the Times.
"Privacy was a principle he took very seriously," she said.
It was his emphasis on privacy that allowed Regan to gain entrance to the behind-the-scenes lives of rock stars and celebrities where most journalists were forbidden.
"We trust him," said James Taylor in an afterword to Regan's book of photography, "All Access," published last year. "We can be ourselves around him. He is one of us."
When asked what makes a photograph special, Regan told online fan magazine "Culture Brats," it's the ability to capture unique images.
"If you're able to capture an image that nobody else has, then that's what makes the image important; that's what people are interested in," he said. "You see hundreds of photographs of rock artists on stage, but do you see them on their plane? Do you see them at home? Do you see them backstage? And those are the things that I always wanted to do."