Steinbeck's contacts with leftist authors, journalists, and labor union figures may have influenced his writing and he joined the League of American Writers, a Communist organization, in 1935. Steinbeck was mentored by radical writers Lincoln Steffens and his wife Ella Winter. Through Francis Whitaker, a member of the Communist Party USA’s John Reed Club for writers, Steinbeck met with strike organizers from the Cannery and Agricultural Workers' Industrial Union. In 1939, he signed a letter with some other writers in support of the Soviet invasion of Finland and the Soviet-established puppet government.
Documents released by the Central Intelligence Agency in 2012 indicate that Steinbeck offered his services to the Agency in 1952, while planning a European tour, and the Director of Central Intelligence himself, Walter Bedell Smith, was eager to take him up on the offer. What work, if any, Steinbeck may have performed for the CIA during the Cold War is unknown.
Steinbeck was a close associate of playwright Arthur Miller. In June 1957, Steinbeck took a personal and professional risk by standing up for him when Miller refused to name names in the House Un-American Activities Committee trials. Steinbeck called the period one of the "strangest and most frightening times a government and people have ever faced."
In 1967, when he was sent to Vietnam to report on the war, his sympathetic portrayal of the United States Army led the New York Post to denounce him for betraying his liberal past. Steinbeck's biographer, Jay Parini, says Steinbeck's friendship with President Lyndon B. Johnson influenced his views on Vietnam. Steinbeck may also have been concerned about the safety of his son serving in Vietnam.
Steinbeck complained publicly about government harassment. Thomas Steinbeck, the author's eldest son, said that J. Edgar Hoover, director of the FBI at the time, could find no basis for prosecuting Steinbeck and therefore used his power to encourage the U.S. Internal Revenue Service to audit Steinbeck's taxes every single year of his life, just to annoy him. According to Thomas, a true artist is one who "without a thought for self, stands up against the stones of condemnation, and speaks for those who are given no real voice in the halls of justice, or the halls of government. By doing so these people will naturally become the enemies of the political status quo."
In a 1942 letter to United States Attorney General Francis Biddle, John Steinbeck wrote: "Do you suppose you could ask Edgar's boys to stop stepping on my heels? They think I am an enemy alien. It is getting tiresome." The FBI denied that Steinbeck was under investigation.