Rumpled by habit, limping on old knees, smiling faintly after a night of celebration, Chuck Feeney stepped out of a building on Park Avenue Monday night and vanished, carried away on a river of passing strangers who knew nothing about him. Perfectly disguised as an ordinary man, Mr. Feeney, one of the most generous and secretive philanthropists of modern times, had dropped from sight once again. It is a skill he mastered over decades.

Last year, the foundation Mr. Feeney created, the Atlantic Philanthropies, gave $458 million in grants around the world, more than any United States charity except two, the Ford and the Bill and Melinda Gates Foundations. Atlantic, and small predecessors also started by Mr. Feeney, have given $4 billion since 1982; the plan is to give away the remaining assets — now $4 billion, but growing every day — by 2017.

Despite this record, Mr. Feeney is little known, a result of the web of intrigues that he fashioned to disguise his identity, his wealth and his giving. Atlantic does not appear in the annual rankings of the biggest American philanthropies because it was set up in Bermuda, to avoid the disclosures required in the United States. A rare glimpse of Mr. Feeney’s story emerged a decade ago during a business dispute, but he quickly disappeared from the news.

Now, however, Mr. Feeney, who is 76 years old and grew up in Elizabeth, N.J., is stepping out from behind his veil. He cooperated with a biographer, the journalist Conor O’Clery, whose book, “The Billionaire Who Wasn’t,” is being published by PublicAffairs. In it, he describes how Mr. Feeney and his partners went into business nearly 50 years ago selling five-pack boxes of liquor to American sailors in ports around Europe, and expanded into a worldwide empire of duty-free airport shops — often one quick step ahead of police or immigration authorities.