Marshall was a correspondent for Reuters for 17 years, covering political upheaval in Thailand and the conflicts in Iraq, Afghanistan and Pakistan. In 2000, he was named Reuters' Deputy Bureau Chief in Bangkok. He was Reuters' Baghdad bureau chief from 2003 to 2005 as a violent insurgency gripped Iraq, and was Reuters' managing editor for the Middle East from 2006 to 2008. From 2008 he was based in Singapore as a political risk analyst and emerging markets editor. He left Reuters in June 2011 when the agency refused to publish a set of articles about Thailand's monarchy he authored based on his analysis of leaked U.S. diplomatic cables.
In June 2011 Marshall announced he had resigned from Reuters to publish a set of stories about Thailand that the news agency had refused to run. Later the same month he published the material himself. Entitled "Thailand's Moment of Truth", his study analysed the role of the monarchy in Thai politics and included references to hundreds of leaked U.S. diplomatic cables. The cables were also later released by WikiLeaks. Thailand has harsh lese majeste laws that criminalize criticism of the royal family, and journalists covering the country have tended to follow a policy of self-censorship, refraining from any comment on the monarchy that could be deemed critical. Marshall's study, usually referred to by its Twitter hashtag #thaistory, used evidence from the cables to argue the monarchy played a central political role in Thailand which had never been properly reported.
In an article for the Independent newspaper, Marshall noted that his publication of #thaistory meant he would face jail if he ever returned to Thailand, and that he understood Reuters' refusal to publish the material given the risks to its staff and business in Thailand if it offended the monarchy. Reuters gave a different explanation, telling The Times and The Independent that the story didn't "work" in the format in which it was delivered, that they had questions regarding length, sourcing, objectivity, and legal issues, and that Marshall "was not participating in the normal editing process."
Marshall's #thaistory generated significant comment and debate. Nicholas Farrelly, a fellow at the Australian National University, wrote that the initial instalments published "have quickly become online sensations", adding "his insights will reverberate in Thai analytical circles for many years to come". Joshua Kurlantzick, Southeast Asia fellow at the Council of Foreign Relations, said Marshall's work was "perhaps the biggest bombshell of reportage on Thailand in decades". Graeme Dobell of the Lowy Institute for International Policy described #thaistory as "journalism of the highest order" and Pavin Chachavalpongpun of the Institute of South East Asian Studies wrote: "Marshall has undoubtedly helped push the boundaries much further as one looks at the present state of the Thai monarchy." Richard Lloyd Parry, Asia editor for The Times newspaper, said #thaistory was "a profound study, beyond mere journalism".
The Thai authorities have a policy of not officially acknowledging the existence of controversial Wikileaks cables, and so did not comment on #thaistory, but Thanong Khanthong, managing editor of the generally pro-establishment Nation newspaper, claimed it was part of an international plot to destabilize Thailand.
Marshall has done extensive research into the mysterious shooting of Ananda Mahidol, King Rama VIII of Thailand, on 9 June 1946. He argues that the evidence overwhelmingly suggests Bhumibol Adulyadej killed his brother, probably accidentally, and this was covered up to enable Bhumibol to become king.
Marshall's book A Kingdom in Crisis was published by Zed Books in October 2014.
Marshall reported Thai King Bhumipol's death on 13 October 2016, hours before the official announcement by the royal palace.