Barnea began his journalism career at university, writing for the student newspaper Pi-Ha'aton. In 1967-1982, he worked for the newspaper Davar, becoming the paper's correspondent in Washington, D.C. Later he founded and edited a weekly paper Koteret Rashit. Since 1989, Barnea has been a staff writer for Yedioth Ahronoth. He wrote for Ha'Ayin HaShevi'it from 1996 to 2008.
Barnea was awarded the Sokolov Prize for journalism in 1981. In a survey in 1998, he was voted one of the most influential journalists in Israel. In 2007, he won the Israel Prize in the sphere of communications. The judges who awarded the prize said: "Nahum Barnea is a journalist who almost every young writer aspires to emulate." They added, "Barnea always insists on 'being there,' close to events, even in places of social tension, even in wars and on days of terror attacks, even when his presence places his life in danger." On 29 March 2007, he accepted an award from the president of Tel Aviv University for "his achievements and his unique contribution to the profession of journalism in Israel.
According to Barnea, Israeli journalists who fail to criticize Arab terrorism fail the--what Barnea coined--Lynch Test. According to Kenneth Levin, this is a "rare instance of Israeli media self-scrutiny". This term was first used after the 2000 Ramallah lynching, in which an Arab mob beat to death two Israeli reservists who had mistakenly entered Ramallah.
In 2007, Barnea was criticized for defending former Prime Minister Ehud Olmert while he was being investigated on suspicions of corruption. Independent journalist Yoav Yitzhak claimed that Barnea did not shy away from publishing statements he knew were untrue.
In January 2011, MK Shelly Yachimovich objected to Barnea's insinuation that women might be responsible for turning themselves into sex objects by wearing provocative clothing, effectively inviting rape. When Barnea drew attention to a photograph of Yachimovich jogging on the beach in shorts, Yachimovich denied that her outfit was provocative and said that anything less than a hijab apparently stimulated Barnea's fantasies. Yachimovich also criticized Barnea for his "tender" interview with former President Moshe Katzav, after his conviction for rape, and for suggesting that the victims were to blame. Barnea subsequently apologized for his remarks.
Barnea has published two collections of his articles, ("They shoot and cry") and ("Days of Netanyahu"). In 2006, he published Backchannel: Bush, Sharon and the Uses of Unilateralism.