Some of Arnett's early days in journalism were in Southeast Asia, particularly Bangkok. He started out running a small English-language newspaper in Laos in 1960. Eventually, he made his way to Vietnam where he was a reporter for the Associated Press. He worked with other AP staff in their Saigon office writing a number of important articles, such as"Death of Supply Column 21", which attracted the ire of the American government. In July 1963, he was punched in the nose by South Vietnamese undercover police while covering Buddhist protests.

He went on dozens of missions with troops, including during the traumatic battle of Hill 875, in which a detachment went to try to rescue another unit of soldiers that was stranded in hostile territory. They themselves were nearly killed during the rescue. In September 1972 he accompanied a group of U.S. peace activists, including William Sloane Coffin and David Dellinger, to Hanoi, North Vietnam to bring three prisoners of war back to the United States.

Arnett got into trouble for writing in an unvarnished manner when trying to report the stories of ordinary soldiers and civilians. Arnett's writing often was perceived as negative. General William Westmoreland and president Lyndon B. Johnson and other people in power had battles with the AP over trying to get Arnett removed from his assignment.

Arnett's most famous act of reporting from the Vietnam War was a story published on February 7, 1968, about the provincial capital B?n Tre: "'It became necessary to destroy the town to save it,' a United States major said today. He was talking about the decision by allied commanders to bomb and shell the town regardless of civilian casualties, to rout the Vietcong." The quote was distorted in subsequent publications, eventually becoming the more familiar, "We had to destroy the village in order to save it." The accuracy of the original quote, and its source, have often been called into question. Arnett never revealed his source, except to say that it was one of four officers he interviewed that day. US Army major Phil Cannella, the senior officer present at B?n Tre, suggested that the quote might have been a distortion of something he said to Arnett. The New Republic at the time attributed the quote to US Air Force major Chester L. Brown. In Walter Cronkite's 1971 book, Eye on the World, Arnett reasserted that the quote was something "one American major said to me in a moment of revelation."

Arnett was one of the last reporters in Saigon after its capture by the North Vietnamese Army, and met with NVA soldiers who showed him how they had come into the city.

Arnett was the writer of the 1980 mini-series documentary - The Ten Thousand Day War.

Following the Soviet Invasion of Afghanistan, Arnett was working for Parade and entered Afghanistan with Mujahideen guides illegally from Pakistan, while dressed as natives. They continued on to a Jalalabad hideaway of approximately fifty rebels. The trip came to an end when Healy fell into the Kunar river, ruining the pair's cameras. Later, Arnett would recount the story to Artyom Borovik who was covering the Soviet side of the war.

Arnett worked for CNN for 18 years ending in 1999. During the Gulf War he became a household name worldwide when he became the only reporter with live coverage directly from Baghdad. His dramatic reports were often given with air raid sirens blaring and the sound of Baghdad bomb explosions in the background. Together with two other CNN journalists, Bernard Shaw and John Holliman, Arnett brought continuous coverage from Baghdad for the 16 initial intense hours of the war (January 17, 1991). Although 40 foreign journalists were present at the Al-Rashid Hotel in Baghdad at the time, only CNN possessed the means to communicate to the outside world. Soon the other journalists left Iraq, including the two CNN colleagues, which left Arnett as the sole reporter remaining there.

His reports on civilian damage caused by the bombing were not received well by the coalition war administration, who by their constant use of terms like "smart bombs" and "surgical precision" had tried to project an image that civilian casualties would be at a minimum. White House sources would later state that Arnett was being used as a tool for Iraqi disinformation and CNN received a letter from 34 Members of the United States Congress accusing Arnett of "unpatriotic journalism".

Two weeks into the war, Arnett was able to obtain an uncensored interview with Saddam Hussein. The Gulf War became the first war to be seen truly live on TV, and Arnett was in many ways the sole player reporting from the "other side" for a period of five weeks.

About halfway through the war the CIA approached Mr. Arnett. They believed that the Iraqi military was operating a high-level communication network from the basement of the Al Rasid Hotel, which is where Mr. Arnett and a few others from CNN were staying. The CIA wanted him out so the Air Force could bomb the hotel, but Mr. Arnett refused. He said he had been given a tour of the hotel and denied there was such a facility.

One of Arnett's most controversial reports during the Gulf War was a report on how the coalition had bombed a baby milk factory. Shortly after the report, an Air Force spokesman stated "Numerous sources have indicated that [the factory] is associated with biological warfare production". Later the same day, Colin Powell stated "It was a biological weapons facility, of that we are sure". White House spokesman Marlin Fitzwater stated "That factory is, in fact, a production facility for biological weapons" and "The Iraqis have hidden this facility behind a façade of baby-milk production as a form of disinformation."

The image of a crudely-made hand-painted sign reading "Baby Milk" in English and Arabic in front of the factory, and a lab coat dressed in a suit containing stitched lettering reading "BABY MILK PLANT IRAQ" only served to further the perception that purportedly civilian targets were simply being made to look like that by Saddam Hussein, and that Arnett was duped by the Iraqi government. The sign appeared to have been added by the Iraqis before the camera crews arrived as a cheap publicity ploy. Newsweek called the incident a "ham-handed attempt to depict a bombed-out biological-weapons plant near Baghdad as a baby-formula factory."

Arnett remained firm. He had toured the plant in the previous August, and was insistent that "Whatever else it did, it did produce infant formula". Described as being a veritable fortress by the Pentagon , the plant, Arnett reported, had only one guard at the gate and a lot of powdered baby milk. "That's as much as I could tell you about it ... [I]t looked innocent enough from what we could see." A CNN camera crew had been invited to tour this plant in August 1990. They videotaped workers wearing new uniforms with lettering in English reading, "Iraq Baby Milk Plant".

Interviewed later, Michel Wery, the plant's French contractor who helped build it, gave an interview in which he stated that the plant was producing solely baby milk when it started up in 1979, and was not equipped to breed pathogens. The plant closed in 1980, he said, when the last French technicians working for his company left Baghdad. Wery said he had heard that production had restarted after the United Nations embargo put in place in the fall of 1991, but he doubted whether that was possible after a 10-year lull. Two dairy technicians had been in the plant at least four times since to make repairs; one stated that, during a visit in May 1990, it was all normal dairy equipment and that the plant was actually canning milk powder. The suspicious uniform stitching was actually part of the original uniforms supplied by the French, and in fact the footage showing the uniforms was shot in August 1990. Part of the problem in reconciling the various U.S. and foreign accounts is that administration officials said they were constrained by security considerations from revealing exactly how they knew about the plant. At the same time, the New Zealand technicians and the French builder were not at the plant after May and cannot be certain of what happened after their departure.

White House reports diverged at this time. One official claimed that the plant was converted in 1990. Another claimed that it was a "backup" bioweapons facility, which had not yet been converted. A third said that it was not a bioweapons facility, but that it was used to make items crucial to bioweapons research; all three claimed insider information. In a confidential memo from December 1992, a State Department employee discussed the issue of the plant and reported that there were no hidden chambers or inappropriate machinery, and that it appeared to be a perfectly normal factory for producing powdered milk.

The plant had undergone security modifications since May 1990. Amongst these were camouflage paint on all the buildings in the complex, a security fence, and the positioning of two SA-2 Surface-to-air missile batteries. In addition, the Iraqis had claimed that they were getting powdered milk for the plant from Nestlé, but Nestlé said that was false. They said they had supplied no products to this plant.

Colin Powell gave the president a briefing a week before the plant was bombed. Powell told President Bush that intelligence based from agents inside Iraq stated that the Iraqis had altered the plant into a biological weapons plant.

The Iraq Survey Group visited the facility in May 2004 and found that it was inoperable and had been out of operation for some time prior to the invasion. The plant was searched extensively and no evidence was found of WMD production, although the production facilities and factory floor were littered with remnants of baby milk production, including large piles of powdered baby milk that had congealed into solid masses.

In 1998 Arnett narrated a joint venture between CNN and Time magazine, NewsStand, which described what he called "Operation Tailwind".

The report, titled The Valley of Death, claimed that the United States Army had used Sarin against a group of deserting U.S. soldiers in Laos in 1970. The report was expressly approved by both CNN Chairman Tom Johnson and CNN President Rick Kaplan. In response, The Pentagon commissioned another report contradicting CNN's. CNN subsequently conducted its own investigation which concluded that the "journalism [in the Valley of Death] was flawed" and retracted the story. Three or more of the individuals responsible for the "flawed" report were fired or forced to resign. Arnett was reprimanded.

The co-producers of the report, April Oliver and Jack Smith, were dismissed. They sued Time Warner, the parent company of CNN, claiming they had been wrongfully fired and Time Warner ultimately paid millions of dollars to settle their law suits, along with other suits brought by military personnel who claimed to have been libeled in the Oliver/Smith report. Senior producer Pam Hill and others resigned. Oliver was later quoted by the World Socialist Web Site (International Committee of the Fourth International) as saying that:

In December 2001 and January 2002, Arnett broadcast exclusive high definition television reports from Afghanistan of the U.S. invasion of Afghanistan for the then new HDNet network.

On assignment for NBC and National Geographic, Arnett went to Iraq in 2003 to cover the U.S. invasion. After a press meeting there he granted an interview to state-run Iraq TV on March 31, 2003, in which he stated:

When Arnett's remarks sparked a "firestorm of protest", NBC initially defended him, saying he had given the interview as a professional courtesy and that his remarks were "analytical in nature". A day later, though, NBC, MSNBC and National Geographic all severed their relationships with Arnett.

In response to Arnett's statement on Iraqi TV, the corporation stated:

Arnett responded:

Later that day, Arnett was hired by the British tabloid, The Daily Mirror, which had opposed the war. A couple of days later he was also assigned to Greek television channel NET television, and Belgian VTM.

In 1964 Arnett married a Vietnamese woman, Nina Nguyen; they had two children, Elsa and Andrew. In 1983 Nina and Peter separated. They divorced more than 20 years later.

Elsa Arnett attended Stuyvesant High School in New York and Harvard University. After graduating she went into journalism, became a reporter, worked for several months on The Washington Post as an intern and then joined The Boston Globe. Elsa Arnett is married to a former White House lawyer John Yoo.