[4][5] He was among survivors of the Canal Hotel Bombing in Iraq during which de Mello was killed with 20 other members of his staff on 19 August 2003.
[6][7] Clapham has worked as Special Adviser on Corporate Responsibility to High Commissioner for Human Rights Mary Robinson, and Adviser on International Humanitarian Law to Sergio Vieira de Mello, Special Representative of the UN Secretary-General in Iraq.
In 2015, Clapham was among two international lawyers whose legal opinion Amnesty International commissioned to reach the conclusion that the United Kingdom was breaching the law by allowing the sales of weapons to Saudi Arabia despite knowing about their use to bomb civilians.
[12] His book The 1949 Geneva Conventions: A Commentary (Oxford University Press; co-edited with Paola Gaeta, and Marco Sassóli) won a Certificate of Merit from the American Society of International Law for "a preeminent contribution to creative scholarship" in 2017.
[13] Clapham is often cited in the news media for his expertise in relation to the United Nations' efforts to hold perpetrators of war crimes accountable and uphold human rights law.