[6][7] He was the son of an Englishman Lieutenant Colonel Benjamin Havilland Churchill Le Mesurier, of the Royal Marines, and his Swedish wife, Ewa.

[3] From 2008 to 2012,[13][better source needed] he worked as an urban security expert for Good Harbor Consulting,[17] chaired by Richard A. Clarke, former U.S. counter-terrorism official who later accused George W. Bush of committing war crimes.

[18][17] From 2012 to 2014, Le Mesurier worked for the UAE consultancy Analysis, Research, and Knowledge (ARK),[19] which stated its goal was to "help realise the legitimate political, social and economic aspirations of conflict-affected communities".

[20] In 2013, with the Turkish NGO AKUT Search and Rescue Association, ARK started training non-governmental Syrian civil defence teams in Turkey, funded by the UK, U.S. and Japanese governments and managed by Le Mesurier.

[21][22][23] From 2011, Le Mesurier worked for the UAE consultancy Analysis, Research, and Knowledge (ARK), an organisation created by Alistair Harris, which was involved multiple projects in Syria.

[15] It continued to train and support Syrian volunteers in what had developed into the White Helmets (founded in 2013) involved in emergency response, including the search and rescue of bombed buildings, and medical evacuation.

[8][27][28] "If you make the decision to risk your life, to save other people, it goes against radicalization", Le Mesurier told di Giovanni in an article for Newsweek in 2016.

[36] Ben Nimmo, of the social media analysis company Graphika, found that such claims began around 2015 with the involvement of Russian forces in the War, and increased after Syrian government forces along with their Russian allies began the Siege of Aleppo in late 2016 with their targeting of hospitals, a potential war crime, which the White Helmets witnessed and were by now able to provide video evidence.

[37] A week before Le Mesurier died, he was accused on Twitter by Russia's Ministry of Foreign Affairs official Maria Zakharova, without evidence, of being a former MI6 agent with "connections to terrorist groups", including al-Qaeda.

[6][41] On 11 November 2019, Le Mesurier was found dead in the street at 4:30 in the morning (1:30 GMT) in the Kemankeş Kara Mustafa Paşa neighbourhood of Beyoğlu, Istanbul, as a result of what appeared to have been a fall from his balcony.

[8] Later The Times reported that the Turkish police were treating the death as suicide, based on information from Le Mesurier's wife and his recent medical history, and that no forensic, autopsy or CCTV evidence indicated otherwise.

A toxicology report, according to a private Turkish broadcaster NTV, found signs of sleeping pills in Le Mesurier's body matching the statement from his widow.

[53] While working on the operation to evacuate White Helmet volunteers from Syria in July 2018, Le Mesurier took $50,000 in cash from Mayday, of which he only used $9,200, instructing his office to offset the remainder against his salary.

In 2019, a Dutch auditing company, SMK, questioned what had happened to the sum of money, and met with Le Mesurier in Istanbul on 7 November (three days before his death), causing him considerable concern; he offered his resignation over the issue.

[35][34] Mayday's new financial director raised a number of issues around accounting practices at the Foundation but most were dismissed in May 2020 by forensic audit experts from Grant Thornton, which came to a conclusion that "the key finding of our investigation of the flagged transactions leads us to believe that there is no evidence of misappropriation of funds.