Cook was born in Point Pleasant, New Jersey, and grew up in a house on Bay Avenue near the border with Bay Head. On his mother’s side, he was descended from an old New Jersey family, the Comptons. He graduated from Rutgers University in 1932.

Cook began his career in journalism at the Asbury Park Press in New Jersey. He later wrote for the New York newspaper, the World-Telegram, focusing on crime reporting. He uncovered the confession of John Francis Roche in the murder case of Navy sailer Edward S. Bates, which freed Paul A. Pfeffer, who had been convicted of the murder.

While editor of the weekly New Jersey Courier in Lakewood, New Jersey, he covered the 1937 Hindenburg disaster. Having witnessed the airship flying overhead at Toms River, New Jersey, he first wrote about its anticipated safe arrival at the Lakehurst Naval Air Station, then had to quickly rewrite the story after getting to the crash site while the ship was still in flames. A few hundred copies of the earlier edition, with the wrong story, were already on their way to news stands, "so I knew I had to collar them and get them back," Cook said.

Though conservative in many respects, Cook wrote a number of articles for The Nation magazine, and took positions usually identified with the left. For instance, he opposed the death penalty, taking the position that it was cruel and didn't deter crime. He was also highly critical of the FBI, the CIA, and the Alger Hiss perjury conviction, as well as oil companies and defense contractors. His writing made him the target of FBI investigations against him.

Cook's 1964 book, Goldwater: Extremist on the Right, initiated a series of events which in the end led to the Supreme Court decision in what is known as the Red Lion case: After the book appeared, Cook was attacked by conservative evangelist Billy James Hargis on his daily Christian Crusade radio broadcast, on WGCB in Red Lion, Pennsylvania. Cook sued, arguing that under the FCC's Fairness Doctrine he was entitled to free air time to respond to the attack. Red Lion Broadcasting challenged the constitutionality of the doctrine, and the case went to the Supreme Court in 1969, with the Court ruling unanimously that the Fairness Doctrine was constitutional.

Cook had written four articles for The Nation by the time then editor Carey McWilliams asked Cook to write an article about the perjury case of Alger Hiss. Cook did not want to do the article, thinking Hiss was "guilty as hell." After two more requests by McWilliams for Cook to do the article, McWilliams said, "Look, I have a proposition to make you. I know how you feel about the case, but I've talked to a lot of people who I trust. They say if anybody looked hard at the evidence they'd have a different opinion. You're known as a fact man. Will you do this for me? No obligation. Will you at least look at the facts?" Cook decided that, as a good journalist, he was obligated to look at the facts and see where they took him.

The September 21, 1957 issue of The Nation was dedicated entirely to Cook's investigation of the Hiss case, which was called, "Hiss: New Perspectives on the Strangest Case of our Time." In the article Cook wrote for The Nation, he ultimately was of the opinion that Hiss was not guilty of the accusations made by Whittaker Chambers who accused Hiss of being a Soviet spy while working for the State Department.

Cook expanded on the article and wrote a book called, The Unfinished Story of Alger Hiss (Morrow, 1957). Cook continued to believe Hiss was innocent up to his death.

This is an incomplete list that doesn’t include all the nonfiction written for children and young adults, his fiction and his works published in magazines.

• ”The Unfinished Story of Alger Hiss”, Morrow, 1958.
• "The Warfare State," Macmillan, 1962.
• ”What Manner of Men: Forgotten Heroes of the Revolution”, Morrow, 1959.
• "Rallying a Free People: Theodore Roosevelt", Kingston House, 1961.
• ”The FBI Nobody Knows”, Macmillan, 1964.
• ”Barry Goldwater: Extremist of the Right", Grove, 1964.
• ”The Corrupted Land: The Social Morality of Modern Americans”, Macmillan, 1966.
• ”The Secret Rulers: Criminal Syndicates and How They Control the U.S. Underworld”, Duell, Sloan & Pearce, 1966.
• ”The Plot against the Patient”, Prentice-Hall, 1967.
• ”What So Proudly We Hailed”, Prentice-Hall, 1968.
• ”The Nightmare Decade: The Life and Times of Senator Joe McCarthy”, Random House, 1971.
• ”The Army-McCarthy Hearings, April-June, 1954: A Senator Creates a Sensation Hunting Communists”, Franklin Watts, 1971.
• ”The Rise of American Political Parties”, Franklin Watts, 1971.
• ”The Cuban Missile Crisis, October, 1962: The U.S. and Russia Face a Nuclear Showdown”, Franklin Watts, 1972.
• ”The Muckrakers: Crusading Journalists Who Changed America”, Doubleday, 1972.
• ”The U-2 Incident, May, 1960: An American Spy Plane Downed over Russia Intensifies the Cold War”, Franklin Watts, 1973.
• ”Dawn over Saratoga: The Turning Point of the Revolutionary War”, Doubleday, 1973.
• ”Mafia”, Fawcett, 1973.
• ”American Political Bosses and Machines”, Franklin Watts, 1973.
• ”The Pinkertons”, Doubleday, 1974.
• ”Lobbying in American Politics”, Franklin Watts, 1976.
• ”Privateers of `76", illustrated by William L. Verrill, Jr., Bobbs-Merrill, 1976.
• ”Julia's Story: The Tragedy of an Unnecessary Death”, Holt, 1976.
• ”Mob, Inc.”, Franklin Watts, 1977.
• ”The Ku Klux Klan: America's Recurring Nightmare”, Messner, 1980.
• ”The Crimes of Watergate”, Franklin Watts, 1981.
• ”The Great Energy Scam: Private Billions vs. Public Good”, Macmillan, 1982.
• ”Maverick: Fifty Years of Investigative Reporting” (autobiography), introduction by Studs Terkel, Putnam, 1984.