Callahan is best known for his 2004 book, The Cheating Culture, a nonfiction work on unethical behavior in American society. Since its publication, The Cheating Culture has been reviewed or discussed in numerous newspapers and magazines. The Los Angeles Times called The Cheating Culture "a breathtaking book," while Esquire proclaimed it a "damning and persuasive critique of America's new economic life." In a profile in The New York Times, Chris Hedges called Callahan "a new liberal with old values." Callahan has appeared on hundreds of radio and television programs to discuss The Cheating Culture. He has also lectured widely on the book to business groups and university audiences, frequently as a keynote speaker.

In addition to The Cheating Culture, Callahan is the author of six other books. These include his 2006 book, The Moral Center, which examines how a market-based economy, i.e. capitalism, with its focus on pursuing self interest, undermines the moral fabric of U.S. society. Callahan argues that powerful market forces have eroded family life, led to a more crass and violent media, fanned a new wave of criminality among the upper classes, and undermined the value of work. The American Prospect has called The Moral Center "fresh and provocative." Callahan is also author of the 2002 book Kindred Spirits, a history of the Harvard Business School Class of 1949. In an interview about the book with The New York Times, Callahan contrasted this earlier group of business leaders, many of whom frowned on conspicuous consumption, with the pervasive greed and corruption found in contemporary corporate America. USA Today called the book "intriguing" and "incredibly detailed." Before writing his recent books on ethics, morality, and business, Callahan published three books on U.S. foreign policy, including Dangerous Capabilities, a biography of Paul Nitze, and Unwinnable Wars, a study of U.S. involvement in such ethnic conflicts as the wars in Bosnia, Rwanda, Lebanon, and Biafra. In addition to his books, Callahan's many articles have appeared in such publications as The Washington Monthly, The Nation, The New York Times, The American Prospect, and Commonweal.

Callahan co-founded Demos in 1999. Demos is a nonpartisan, non-profit public policy organization which seeks to create a fairer, more democratic America. Headquartered in New York City, with offices in Boston, Washington, D.C., and Austin, Demos publishes frequent research reports, as well as supporting 15 fellows who write books, articles, and studies. It hosts frequent public events, both in New York and in other cities around the United States. Demos staff and fellows are frequently quoted in the media on topics such as credit card debt, the squeeze on the middle class, voting problems, and the rise of income inequality. In 2010, Demos had a budget of $5.5 million.