During the 1920s and early 1930s, McWilliams joined a loose network of mostly Southern California writers that included Robinson Jeffers, John Fante, Louis Adamic, and Upton Sinclair. His literary career also benefited greatly from his relationships with Mary Austin and H.L. Mencken, who provided an outlet for his early journalism and floated the idea for his first book, a 1929 biography of popular writer and sometime Californian Ambrose Bierce.
The Depression and rise of European fascism in the 1930s radicalized McWilliams. He began working with numerous left-wing political and legal organizations, including the American Civil Liberties Union and the National Lawyers Guild, and wrote for Pacific Weekly, Controversy, The Nation, and other progressive magazines. He also continued to represent workers in and around Los Angeles, helped organize unions and guilds, and served as a trial examiner for the newly formed National Labor Relations Board.
His first bestseller, Factories in the Field, appeared in 1939 and ranks among his most enduring works. Published within months of John Steinbeck's The Grapes of Wrath, it examines the lives of migrant farm workers in California and condemns the politics and consequences of large-scale agribusiness. Shortly before its publication, McWilliams accepted an offer from incoming governor Culbert Olson to head California's Division of Immigration and Housing. Over his four-year term, he focused on improving agricultural working conditions and wages, but his hopes for major reform deteriorated with the advent of World War II.
McWilliams left his government post in 1942, when incoming governor Earl Warren promised campaign audiences that his first official act would be to fire McWilliams. He was a sharp critic of Warren, whom he described as "the personification of Smart Reaction", but he became an enthusiastic admirer after Warren joined the U.S. Supreme Court the following decade. No such conversion occurred in his attitude toward another California politician, Richard Nixon, whom McWilliams described in 1950 as "a dapper little man with an astonishing capacity for petty malice".
After leaving state government, McWilliams continued to write prolifically. He turned his attention to issues of racial and ethnic equality, writing a series of important books (including Brothers Under the Skin, Prejudice, North from Mexico, and A Mask for Privilege) that dealt with the treatment of immigrant and minority groups. He also produced two regional portraits, Southern California Country: An Island on the Land (1946) and California: The Great Exception (1949), which many aficionados still regard as the finest interpretive histories of those areas. Decades after its publication, Southern California Country inspired Robert Towne's Oscar-winning original screenplay for the movie Chinatown (1974).
Witch Hunt (1950) was an early attempt to combat McCarthyism, which McWilliams considered a grave threat to civil liberties and healthy politics. Although he was never a member of the Communist Party, he was a frequent target of anti-Communist attacks. In the 1940s, he was called before the Committee on Un-American Activities in California, and FBI director J. Edgar Hoover placed him on the Custodial Detention List, making him a candidate for detention in case of national emergency--even though McWilliams was serving in state government at the time.
McWilliams's activism took many forms. In the early 1940s, he helped overturn the convictions of mostly Latino youths following the so-called Sleepy Lagoon murder trial. He also helped cool the city's temperature during the Zoot Suit Riots of 1943, when scuffles between military personnel and Latinos spun out of control. Once out of government, he became an outspoken critic of the evacuation and internment of Japanese-American citizens. In 1944, Prejudice was cited repeatedly in a Supreme Court dissenting opinion in Korematsu v. United States, the decision upholding the constitutionality of the internment. Several years later, a group of Los Angeles screenwriters, directors, and producers known as the Hollywood Ten were cited for contempt of Congress after refusing to answer a House committee's questions about Communist Party membership. McWilliams drafted a Supreme Court amicus brief for two of them, John Howard Lawson and Dalton Trumbo. (The Court declined to hear their appeal.)
Though by 1951 a committed Californian, McWilliams moved to New York City to work at The Nation under then editor Freda Kirchwey in attempts to revitalize the magazine during its most difficult period. Taking over as editor in 1955, he stayed through 1975 and is credited with strengthening the magazine's investigative reporting and introducing the ideas of the New Left to more mainstream audiences. He also published the early work of Ralph Nader, Howard Zinn, Theodore Roszak, and Hunter S. Thompson, who credited McWilliams with the idea for his first bestselling book, Hell's Angels: A Strange and Terrible Saga (1966).
Since his death in 1980, McWilliams's critical fortunes have risen steadily. He is now widely regarded as the finest nonfiction writer on California and the state's pre-eminent public intellectual. Reviewing his overall achievement, McWilliams's biographer maintains that he may also be the most versatile American public intellectual of the 20th century.
The American Political Science Association gives an annual Carey McWilliams Award "to honor a major journalistic contribution to our understanding of politics."
Writing for The Nation, as the magazine's editor, McWilliams was the first American reporter to reveal that the CIA was training a group of Cuban exiles in Guatemala for the Bay of Pigs Invasion. His article, titled "Are We Training Cuban Guerrillas?", was published in November 1960, five months before the invasion occurred, during the Eisenhower Administration.
The story was largely ignored by major newspapers like the New York Times and Washington Post. Arthur Schlesinger, an aide to President John F. Kennedy, pressured The New Republic not to run a story about the guerrilla force. Following the failure of the invasion, Kennedy expressed regret that more information about the invasion plan was not published, telling Times reporter Turner Catledge, “If you had printed more about the operation, you would have saved us from a colossal mistake.”