Kahn was born in London, England to an affluent politically conservative Jewish family. Educated in the United States, he attended Phillips Exeter Academy and Dartmouth College, where he was a star athlete. His education exposed him to Shakespeare, and later in life he said that it was the study of King Lear that first awakened in him a sense of injustice. He was Dartmouth Class Poet, graduating in 1932. Married in 1934, he and the former Harriet Warner moved to California, where Kahn tried unsuccessfully to become a Hollywood screenwriter.
After the outbreak of the Spanish Civil War, Kahn agreed to lead an ambulance tour to raise medical relief funds for Loyalist forces fighting against the fascist-supported Franco rebellion. On the tour, Kahn spoke to audiences ranging from the wealthy to the unemployed. It was the height of the Depression and Kahn was deeply affected by the widespread deprivation that he saw. Communists and socialists organized many of the speaking events and impressed Kahn with their idealism. After completing the tour in 1938, he joined the Communist Party of the United States.
With no employment prospects, Kahn accepted a job at Albert Kahn, Inc., but his political activism quickly caused a rupture. A talented orator, he began giving anti-fascist speeches. As he shared his name with his prominent uncle, the publicity caused consternation at the firm. Their concern was heightened by the reality that Henry Ford was the company's largest client, and Ford was engaged in business in Nazi Germany. In a meeting with his uncle and father, the younger Kahn was given a choice: Stop speaking publicly, or resign. He chose the latter option.
He was the American Labor Party candidate in the 1948 elections for New York's 25th congressional district.
Almost immediately Kahn was offered a position as Executive Director of the newly formed American Council Against Nazi Propaganda. Working for a Board of Directors including Helen Keller, Condé Nast, John Gunther, former Ambassador William E. Dodd, and Thomas Mann, Kahn founded The Hour, a syndicated newsletter. In that capacity he engaged in investigative journalism to expose Nazi espionage, sabotage and propaganda operations in the United States. He also investigated the activities of American fascist and pro-fascist groups such as the German-American Bund. The Hour's revelations were widely used in printed media, by radio commentators such as Walter Winchell, and by the War Department, Justice Department and the Office of War Information.
Material obtained by The Hour became the foundation for Kahn's first best-selling book, Sabotage! The Secret War Against America (1942), co-authored with Michael Sayers. Plans by Reader's Digest to print excerpts from the book resulted in the first notations by FBI Director J. Edgar Hoover in Kahn's FBI file: "Can nothing be done to stop this?"
Kahn and Sayers also collaborated on The Plot Against The Peace (1945) and The Great Conspiracy: The Secret War Against Soviet Russia (1946), an international bestseller. In the latter, on the Moscow purge trials, the authors accepted as valid the charges of treason against former Soviet opposition leaders, and the underlying allegation of plots to overthrow the Soviet state, assassinate Lenin, Stalin, Gorky, and others.
Kahn, an outspoken opponent of the Cold War, was blacklisted from mainstream publishing in the late 1940s. Using pre-sales of books to leftist trade unions, he wrote and published High Treason: The Plot Against the People (Lear, 1950), a post-1917 political history of the United States, and The Game of Death: Effects of the Cold War on Our Children (C&K, 1953).
In the early 1950s, Kahn and Angus Cameron, an eminent Little, Brown editor who had recently been blacklisted, formed the publishing firm Cameron & Kahn. In 1955 the firm published False Witness, the confession of former Communist and paid government witness, Harvey Matusow, that he had repeatedly lied under oath. Matusow's announced confession caused a sensation, and the government's response to pending publication of the book was to subpoena Kahn, Cameron and Matusow to appear before a federal grand jury. The publishers were accused of bribing Matusow to falsely assert that he had committed perjury on behalf of the government. After months of hearings and thousands of pages of testimony, the grand jury declined to issue indictments against Cameron or Kahn.
Simultaneously with the grand jury proceedings, Kahn, Cameron and Matusow were subpoenaed to testify before the United States Senate Internal Security Subcommittee, chaired by the Mississippi senator, James Eastland. The purpose of the hearings was to determine whether publication of False Witness was the result of a Communist conspiracy to have Matusow lie in admitting perjury, rather than to assess the origin and consequences of Matusow's admitted perjury.
The story of the book's publication and its aftermath was written by Kahn in the late 1950s, but not published until 1987, eight years after his death (The Matusow Affair, Moyer Bell).
Other books published by Cameron and Kahn included The testament of Ethel and Julius Rosenberg, Seeds of Destruction; the truth about the U.S. occupation of Germany by Cedric Belfrage and The ecstasy of Owen Muir by Ring Lardner.
During the 1950s, Kahn had his passport revoked for refusing to sign the required affidavit stating whether or not he was or had ever been a member of the Communist Party, a requirement ruled unconstitutional by the United States Supreme Court in a case involving noted painter and Kahn friend, Rockwell Kent.
Kahn broke the blacklist in 1962 with publication by Simon & Schuster of the critically acclaimed Days With Ulanova, an intimate portrait of the fabled Bolshoi ballerina. On another trip to Moscow, Kahn met with Soviet leader Nikita Khrushchev in the Kremlin and proposed collaboration with him on the Soviet leader's autobiography. Khrushchev agreed, but was forced from office before the project was launched. Other Kahn books included Smetana and the Beetles (Random House, 1967), a satire of the defection of Stalin's daughter; Joys and Sorrows (Simon & Schuster, 1970), Pablo Casals' memoir as told to Kahn; and The Unholy Hymnal (Simon & Schuster, 1971), a satirical expose of the Credibility Gap of the Johnson and Nixon administrations.
After his death, speculation developed as to whether Kahn had served Soviet intelligence. In 1946 the San Francisco KGB suggested that Kahn be recruited into Soviet espionage. Kahn requested that Julia Older, who worked in the Office of Strategic Services (OSS), obtain information. Elizabeth Bentley stated in her deposition to the FBI that Kahn had furnished information directly to Jacob Golos and herself in 1942 on immigrant Ukrainians hostile to the Soviet Union. During that period, the Soviet Union was an ally of the United States in the war against Nazi Germany. Ukrainian nationalist and pro-fascist organizations were considered by the American government as allies of the Germans, and at the time Kahn shared his investigative findings with the FBI and American military intelligence. Venona project researchers John Earl Haynes and Harvey Klehr speculate Kahn may be code name "Fighter", as referenced in Venona decypt # 247 San Francisco to Moscow, 14 June 1946.
In September 1958, Kahn was called for the final time to testify before the Senate Internal Security Subcommittee. One witness, Fedor Mansvetov, testified that he knew Kahn to be a Soviet spy because "he is following party line" by not referring to East European countries as "satellites". Kahn submitted an affidavit with the committee which charged that "witnesses at your hearings have been repeatedly encouraged to bandy about...grotesque accusations", and included a challenge:
"If I could sue your committee for defamation of character and interference with my work, I would. It might be a good lesson for you. Perhaps you will advise me whether each of your committee members is willing to waive his congressional immunity and assume full personal responsibility for spreading the charges made against me by your witnesses at this hearing. Perhaps just one of you –let us say Senator Eastland –will repeat in public and without congressional immunity the accusation that I am a spy. There seems a peculiar aptness to that popular American saying, 'Put up or shut up.'"
None of the Senators accepted his offer.
He died on September 15, 1979, of a heart attack in Glen Ellen, California.