In 1969 he was a founding editor of Socialist Revolution (which was later renamed Socialist Review and then Radical Society before ceasing publication in 2009). In the 1970s he was a founding editor of the East Bay Voice. He moved to Chicago in December 1976 to become the foreign editor of In These Times, a democratic socialist newsweekly. Judis moved to Washington in 1982. He started writing for The New Republic in 1984, became a contributing editor in 1989, and joined the regular staff in 1995. He quit in December 2014 along with other editors in protest of the owner Chris Hughes' firing of the editor and plan to turn the magazine into a profit-making vehicle.
In 1988, he published a biography, William F. Buckley: Patron Saint of the Conservatives; in 1992, Grand Illusion: Critics and Champions of the American Century; in 2000, The Paradox of American Democracy. In 2002, he published The Emerging Democratic Majority (co-written with political scientist Ruy Teixeira), a book arguing that Democrats would retake control of American politics by the end of the decade, thanks in part to growing support from minorities, women, and well-educated professionals. Its title was a deliberate echo of Kevin Phillips' 1969 classic, The Emerging Republican Majority. The book was named one of the year's best by The Economist. Later in 2015, in an essay The Emerging Republican Advantage  he revised this view as he noted that the long term Democratic Majority had given way to an "unstable equilibrium" between the parties.
In 2004, he published "The Folly of Empire: What George W. Bush could learn from Theodore Roosevelt and Woodrow Wilson," an attempt to put the disastrous American invasion of Iraq in historical context. In 2014 he authored the book Genesis: Truman, American Jews, and the Origins of the Arab/Israeli Conflict in which he discussed the connection between the Israel lobby in the United States and the origin of the modern state of Israel. In 2016, he published The Populist Explosion: How the Great Recession Transformed American and European Politics. This book, which was widely reviewed, analyzed, among other things, the remarkable success of Bernie Sanders and Donald Trump.