Arnett became a reporter for the Associated Press, based in Saigon in the South, in the years when the United States began to get involved in the civil conflict and through the Vietnam War.

On 7 July 1963, in what became known as the Double Seven Day scuffle, he was injured in a widely reported physical altercation between a group of western journalists and South Vietnamese undercover police.

His articles, such as "Death of Supply Column 21," about an event during Operation Starlite in August 1965, resulted in raising the ire of the American government, which had been increasing the number of forces in the region.

In September 1972, Arnett joined a group of U.S. peace activists, including William Sloane Coffin and David Dellinger, on a trip to Hanoi, North Vietnam, to accept three American prisoners of war for return to the United States.

General William Westmoreland, President Lyndon B. Johnson and others in power put pressure on the AP to get rid of or transfer Arnett from the region.

[6]: 259–60 In what is considered one of his iconic dispatches, published on 7 February 1968, Arnett wrote about the Battle of Bến Tre: "'It became necessary to destroy the town to save it,' a United States major said today.

[8] US Army Major Phil Cannella, the senior officer present at Bến Tre, suggested the quotation might have been a distortion of something he said to Arnett.

[9] In Walter Cronkite's 1971 book, Eye on the World, Arnett reasserted the quotation was something "one American major said to me in a moment of revelation.

[6]: 305 Arnett wrote the 26-part mini-series documentary, Vietnam: The Ten Thousand Day War (1980), produced by Canadian Broadcasting Corporation (CBC).

With a contact named Healy, he entered Afghanistan illegally from Pakistan; both men were dressed in traditional clothing as natives and led by Mujahideen guides.

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 (17 January 1991).

Although 40 foreign journalists were present at the Al-Rashid Hotel in Baghdad at the time, only CNN possessed the means — a private phone line connected to neighbouring Amman, Jordan — to communicate to the outside world.

Its spokesmen had emphasized terms such as "smart bombs" and "surgical precision" in their public statements, in an effort to project keeping civilian casualties would be at a minimum.

They believed that the Iraqi military was operating a high-level communication network from the basement of the Al Rashid Hotel, which is where Arnett and other staff from CNN were staying.

"[15][16] In 1998, Arnett narrated a report on the joint venture (between CNN and Time magazine) programme called NewsStand, covering "Operation Tailwind" in Laos in 1970.

The report, titled The Valley of Death, claimed that in 1970, the United States Army had used sarin, a nerve agent, against a group of deserting U.S. soldiers in Laos.

Due to a number of rebuttals claiming the CNN report was flawed, three or more of the individuals responsible were fired or forced to resign.

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.

Arnett was part of the live coverage beginning on 16 January 1991, the start of the Gulf War air campaign, where he and colleagues Bernard Shaw and John Holliman kept broadcasting from their Al-Rasheed Hotel room amid extensive aerial bombing by the Western Coalition forces.