Chambers carried on his espionage activities from 1932 until 1937 or 1938 even while his faith in Communism was waning. He became increasingly disturbed by Joseph Stalin's Great Purge, which began in 1936. He was also fearful for his own life, having noted the murder in Switzerland of Ignatz Reiss, a high-ranking Soviet spy who had broken with Stalin, and the disappearance of his friend and fellow spy Juliet Poyntz in the United States. Poyntz had vanished in 1937, shortly after she had visited Moscow and returned disillusioned with the Communist cause due to the Stalinist Purges.
Chambers ignored several orders that he travel to Moscow, worried that he might be "purged." He also started concealing some of the documents he collected from his sources. He planned to use these, along with several rolls of microfilm photographs of documents, as a "life preserver" to prevent the Soviets from killing him and his family.
In 1938, Chambers broke with Communism and took his family into hiding, storing the "life preserver" at the home of his nephew and his parents. Initially he had no plans to give information on his espionage activities to the U.S. government. His espionage contacts were his friends, and he had no desire to inform on them.
The August 1939 Hitler-Stalin non-aggression pact drove Chambers to take action against the Soviet Union. In September 1939, at the urging of anti-Communist, Russian-born journalist Isaac Don Levine, Chambers and Levine met with Assistant Secretary of State Adolf Berle. Levine had introduced Chambers to Walter Krivitsky, who was already informing American and British authorities about Soviet agents who held posts in both governments. Krivitsky told Chambers it was their duty to inform. Chambers agreed to reveal what he knew on the condition of immunity from prosecution. During the meeting, which took place at Berle's home, Woodley Mansion in Washington, Chambers named 18 current and former government employees as spies or Communist sympathizers. Many names mentioned held relatively minor posts or were already under suspicion. Some names, however, were more significant and surprising: Alger Hiss, his brother Donald Hiss, and Laurence Duggan—who were all respected, mid-level officials in the State Department—and Lauchlin Currie, a special assistant to Franklin Roosevelt. Another person named had worked on a top secret bombsight project at the Aberdeen Proving Grounds.
Berle found Chambers' information tentative, unclear, and uncorroborated. He took the information to the White House, but the President dismissed it, to which Berle made little if any objection. Berle kept his notes, however (later, evidence during Hiss' perjury trials).
Berle notified the Federal Bureau of Investigation (FBI) of Chambers's information in March 1940. In February 1941, Krivitsky was found dead in his hotel room. While police ruled the death a suicide, it was widely speculated that Krivitsky had been killed by Soviet intelligence. Worried that the Soviets might try to kill Chambers too, Berle again told the FBI about his interview with Chambers. Nevertheless, the FBI took no immediate action, in line with the political orientation of the United States, which viewed the potential threat from the USSR as minor, when compared to that of Nazi Germany.
(The FBI did interview Chambers in May 1942 and June 1945, without further action. Only in November 1945, when Elizabeth Bentley defected and corroborated much of Chambers's story, did the FBI begin to take Chambers seriously).
By the time of the Berle meeting, Chambers had come out of hiding after a year and joined the staff of Time Magazine (April 1939). He landed a cover story within a month on James Joyce's latest book, Finnegans Wake. He started at the back of the magazine, reviewing books and film with James Agee and then Calvin Fixx. When Fixx died in October 1942, Wilder Hobson succeeded him as Chambers' assistant editor in Arts & Entertainment. Other writers working for Chambers in that section included: novelist Nigel Dennis, future New York Times Book Review editor Harvey Breit, and poets Howard Moss and Weldon Kees. During this time, a struggle arose between those, like Theodore H. White and Richard Lauterbach, who raised criticism of what they saw as the elitism, corruption and ineptitude of Chiang Kai-shek's regime in China and advocated greater cooperation with Mao's Red Army in the struggle against Japanese imperialism, and Chambers and others like Willi Schlamm who adhered to a staunchly pro-Chiang, anti-communist perspective (and who both later joined the founding editorial board of William F. Buckley, Jr.'s National Review). Time founder Henry R. Luce, who grew up in China and was a personal friend of Chiang and his wife, came down squarely on the side of Chambers to the point that White complained that his stories were being censored, and even suppressed in their entirety, and left Time shortly after the war as a result. With Luce's blessing, Chambers received a promotion to senior editor in September 1943 and was made a member of Time's "Senior Group", which determined editorial policy, in December.
By early 1948, Chambers had become one of the best known writer-editors at Time. First had come his scathing commentary "The Ghosts on the Roof" (March 5, 1945) on the Yalta Conference (in which Hiss partook). Subsequent cover-story essays profiled Marian Anderson, Arnold J. Toynbee, Rebecca West and Reinhold Niebuhr. The cover story on Marian Anderson (December 30, 1946) proved so popular that the magazine broke its rule of non-attribution in response to readers' letters:
Most Time cover stories are written and edited by the regular staffs of the section in which they appear. Certain cover stories, that present special difficulties or call for a special literary skill, are written by Senior Editor Whittaker Chambers."
Chambers was at the height of his career when the Hiss case broke later that year.
During this period, Chambers and his family became Quakers, attending Pipe Creek Friends Meetinghouse near his Maryland farm.