After spending several years as a journalist with The Charleston News and Courier in Charleston, South Carolina, he became a reporter for the Washington Star in 1979. Barnes covered the Supreme Court and the White House for the Star before moving to the Baltimore Sun. He was the national political correspondent at the Baltimore Sun. For ten years from 1985 to 1995, he was senior editor and White House correspondent for The New Republic. He also wrote the "Presswatch" media column for the American Spectator. He was a panelist on the public affairs show The McLaughlin Group from 1988 to 1998, where he was often referred to by the show's host as Freddy "the Beadle" Barnes.
Barnes has made cameo appearances in the Hollywood films Dave, Getting Away with Murder, Independence Day and My Own Private Idaho. He has thrown out the first pitch for a Boston Red Sox baseball game at Fenway Park.
Though having gone to separate high schools, Barnes was a friend of fellow Fox News personality Brit Hume in high school, and at The University of Virginia. Hume had gone to another elite private school, St. Albans.
In Spring of 2007, Barnes appeared as a panelist on Special Report with Brit Hume. In it, he pointed out that there was no scientific consensus on global warming citing, among other things, a difference in the claims by Al Gore's movie and the claims of Intergovernmental Panel on Climate Change United Nations.
BARNES: "There is, no, Mort you've been in Tibet for three weeks, believe me, everyday you were gone some new scientist came out and said he didn't believe in the extravagant tales that are told by people like Al Gore. Al Gore—remember the difference between Al Gore and the Intergovernmental Panel on Climate Change of the United Nations, Al Gore says that over the next century sea level rises 20 feet. This panel on climate change says well maybe 17-23-inches, which I think we can live—look, we don't know whether humans are causing this—we don't even know whether global warming's bad. Me, I like warmer weather."
BARNES: There's a man [Al Gore] who has a fever. That's for sure. Look, it is clear now that there is no scientific consensus on global warming, except for one thing: we know the temperature in the globe increased by 1 degree over the last 100 years. And - and scientists are increasingly embarrassed by the wild exaggerations of Al Gore, where he - he says we're going to get a - the sea level will rise 20 feet, and the U.N. group that's looked into this says it'll be 23 inches. You'll have to admit, there's a significant difference there. And - and I think you're going to increasingly see - well, we have seen, in that New York Times story, scientists increasingly ready to go public to repudiate Al Gore. And yet, you know, you find some Al Gore apologist like yourself, Mort.