This September will mark the jubilee of that masterpiece, the fiftieth year since it appeared in print. It has aged well—preserved in a Mitchell collection published by Pantheon Books (Up in the Old Hotel, 1992), in a film by Stanley Tucci (Joe Gould’s Secret, 2000), and in countless college courses. Joe Gould’s Secret was built to last. “No bent nails,” the editor William Maxwell once observed. “Every word driven, so to speak, all the way into the wood.”

But if Joe Gould’s Secret is well known, Joe Mitchell’s secret is not.

In the spring of 1944—more than a year after Mitchell had profiled Gould—a woman stepped forward to provide the homeless writer with room and board. The woman insisted that she remain anonymous, and arranged for a go-between to give Gould a weekly stipend. It was a benefaction out of the blue, and would, in time, play a pivotal role in his life. Gould was desperate to learn who his patron was. “I’d almost rather know who she is,” he once snapped at Mitchell, “than have the money!” But he never found out.

Mitchell himself learned her identity only in 1959, in conversation with one of the woman’s few confidants. And he dropped a few breadcrumbs into his 1964 article, describing the patron as “a very reserved and very busy professional woman who was a member of a rich Middle Western family and had inherited a fortune and who sometimes anonymously helped needy artists and intellectuals.” But Mitchell revealed nothing more, and took what he knew to his grave. And so, even as Mitchell’s book joined the literary canon, no postscript was added to it—no name ever given to the “professional woman” who had supported its protagonist.

When Mitchell died, he left behind the copious remains of both a career and a collection—a few hundred thousand sheets of paper and a few thousand found objects from the city he had chronicled: buttons, nails, doorknobs, spoons. The papers were given into the care of Sheila McGrath, a former assistant at The New Yorker, whom Mitchell had named as his literary executor. When McGrath died, in September 2012, Mitchell’s elder daughter, Nora Sanborn, then 72, became his literary executor and took possession of his papers, which, she says, were packed into more than 100 cartons.

The next month, Sanborn, a retired probation officer in New Jersey with blue eyes and graying honey hair, took part in a commemoration of Joe Mitchell along the piers of lower Manhattan. I met her on that occasion and asked if she knew who the anonymous patron was. Sanborn said she did not. But she agreed to search the files to see if they might yield a name.

Sanborn was back in New York seven months later, this past spring, for another celebration of her late father. Dressed in a black blouse and black slacks, she sat with some 40 others in a windowed gallery abutting the East River, and looked up at a wiry old man seated on a high wooden chair. He had a white beard and blue eyes and a face that was either tan or sallow. His name was Jack Putnam. He had known Mitchell, and on this misty May day, he began to read aloud a story written by him in 1944, “The Black Clams.” Like almost everything Mitchell wrote, it was true and funny and straight and sacred, devoid of judgment and alight with lists.

As the audience listened to what her father had written, Sanborn held on her lap a folder filled with more of his words: an account of two dinners Mitchell had had in 1959 with a man named John Rothschild, and a letter Rothschild had written years earlier to that woman from a “rich Middle Western family.” The papers were neatly typed and dated. In the upper-right corner of a few of the sheets, Mitchell had scrawled the name “Joe Gould.”