From 1891 to 1900, he was a reporter for the New York Sun and then joined McClure's Magazine, where he gained a reputation as a muckraker for his articles on the conditions of public health in the United States.
In a series of eleven articles he wrote for Collier's Weekly in 1905, "The Great American Fraud", Adams exposed many of the false claims made about patent medicines, pointing out that in some cases these medicines were damaging the health of the people using them. The series had a huge impact and led to the passage of the 1906 Pure Food and Drug Act.
In 1911 the Supreme Court ruled that the prohibition of falsifications referred only to the ingredients of the medicine. This meant that companies were again free to make false claims about their products. Adams returned to the attack and another series of articles in Collier's Weekly, Adams exposed the misleading advertising that companies were using to sell their products.
Adams was a prolific writer, who wrote fiction as well. "Night Bus", one of Adams's many magazine stories, became the basis for the film It Happened One Night. His best-known novel, Revelry (1926), based on the scandals of the Harding administration, was later followed by Incredible Era (1939), a biography of Harding. Among his other works are The Great American Fraud (1906), The Flying Death (1906), The Mystery (1907), with S. E. White, Average Jones (1911), The Secret of Lonesome Cove (1912), The Health Master (1913), The Clarion (1914), The Unspeakable Perk (1916). Our Square and the People in It (1917), Success (1921), Siege (1924), The Harvey Girls (1942), Canal Town (1944), Grandfather Stories (1955), and Tenderloin (1959). The Clarion and Success are studies of modern journalism.
Tenderloin described the battle between Charles H. Parkhurst and Tammany Hall. New York Times reviewer H. I. Brock called the book an "outstanding period piece" and "a worthy finale to a long and varied writing career." Tenderloin was adapted into a 1960 musical with book by George Abbott and Jerome Weidman and songs by Jerry Bock and Sheldon Harnick, the team that had created Fiorello!. Tenderloin ran for 216 performances. New York Times critic Howard Taubman praised the songs, but complained about a "dragging book" and said "The wages of virtue, alas, are largely dullness."
In the 1920s Adams wrote two novels, Flaming Youth and Unforbidden Fruit, dealing with the sexual urges of young women in the Jazz Age: these novels had a sexual frankness that was shocking for their time, and Adams published them under the pseudonym "Warner Fabian" so that his other works would not be tainted by any scandal accruing to these novels. Both of the Warner Fabian novels became best-sellers, and both were filmed: the latter as The Wild Party (not related to a work of the same title by Joseph Moncure March).