Born in New York City, he was the son of architect Ely Jacques Kahn. He attended the Horace Mann School and Harvard University, where he took his B.A. in 1937. He was hired by The New Yorker in 1937 and his first byline appeared there in the April 3, 1937 issue. Before World War II, he was drafted and served in the U.S. Army from 1941 to 1945. His Army experience resulted in The New Yorker publishing thirty-nine of his pieces on Army life; they were later collected in book form.
His long career with the magazine resulted in numerous books on such varied subjects as Coca Cola, Leslie McNair, the Trust Territory of the Pacific, Harvard University, Herbert Bayard Swope, Frank Sinatra, Dwayne O. Andreas of Archer Daniels Midland, and the Postal Inspection Service. However his multi-part series on grain, which was published in book form as "Staffs of Life" in 1985, was criticized by some as an example of the self-indulgent journalism that marked The New Yorker during the 1970s and '80s.
He taught writing at Columbia University from 1974 to 1977.
His book The New Yorker and Me (New York: G.P. Putnam's, 1979) is a diary interspersed with memories of his life, the magazine, and its editor William Shawn--who Kahn calls "The Iron Mouse." His 1987 diary was released as Year of Change: More about the New Yorker and Me (New York: Viking, 1988).