[1] Emmerson's former clients include WikiLeaks founder Julian Assange (in connection with an attempt to extradite him to Sweden);[8] GCHQ whistleblower Katharine Gun (who was prosecuted under the UK's Official Secrets Acts following the 2003 invasion of Iraq);[9][10] and Marina Litvinenko (the wife of former Russian FSB officer Alexander Litvinenko, who was assassinated in London in 2006 using the radioactive isotope Polonium 210).

He produced reports on the counter-terrorism policies of Saudi Arabia,[23][24] Chile,[25][26][27] Sri Lanka,[28][29] Tunisia[30] and Burkina Faso.

[33] He also produced reports on the use of armed drones for counter-terrorism operations in Pakistan, Afghanistan and Palestine,[34][35][36][37][38][39] accountability for the torture of terrorist suspects,[40][41][42][43][44] human rights violations committed by ISIS in Iraq and Syria,[45] electronic surveillance,[46][47] terrorism and migration,[48][49] the impact of national security measures on civil society,[50] and the UN's counter-terrorism sanctions regime.

[56][57] In 2003, Emmerson represented Islamist cleric Abu Qatada, accused of being a member of Al-Qaeda, along with a number of inmates of Belmarsh Prison, at a joint hearing of the Special Immigration Appeals Commission, challenging their indefinite detention, without charge or trial, on national security grounds.

[61] The case subsequently went to the European Court of Human Rights in Strasbourg which upheld the decision of the House of Lords, adding that the secret nature of the procedure was unfair to the accused.

[62] In a subsequent decision arising out of the same case, Emmerson successfully argued in the House of Lords that the detention of terrorist suspects could not be justified by reference to evidence obtained by acts of torture committed abroad by the agents of a foreign state.

[63] Between 2005 and 2012, Emmerson successfully defended Ramush Haradinaj, the former Prime Minister of Kosovo, on war crimes charges at the International Criminal Tribunal for the former Yugoslavia (ICTY) in The Hague.

[11][12][13][14] Emmerson argued a successful judicial review challenge to the British Government's refusal to hold a public inquiry to examine the responsibility of the Russian State for Litvinenko's murder.

[72] Following the publication of the Inquiry report, Emmerson described the murder as an act of "nuclear terrorism" and called on Prime Minister David Cameron to take effective action, saying that a failure to respond would be a "craven" abdication of his responsibilities.

[73] An official spokesperson for the Prime Minister commented: "The conclusion that the murder was authorised at the highest levels of the Russian State is extremely disturbing" and promised to take effective action in response.

[77] He also took up the case of several prominent Catalan politicians and civil society leaders at the UN Working Group on Arbitrary Detention, arising out of their imprisonment in Spain on charges of sedition and rebellion for their part in the 2017 referendum.

[82] During the International Criminal Court investigation in Venezuela, Emmerson represented Nicolás Maduro's government its appeal of the process.

Emmerson was appointed as a Commander of the Order of the British Empire (CBE) in the 2020 Birthday Honours for services to international human rights and humanitarian law.