[3][4] He currently serves as the president of the Capital Punishment Justice Project (formerly Reprieve Australia), an organisation which aims to provide legal representation and humanitarian assistance to those at risk of execution.
[9] McMahon opposes the use of the death penalty internationally, and challenged Australian citizen Van Nguyen's execution by Singapore in 2005, along with Lex Lasry QC.
[10][11] McMahon has represented the Australian citizens Andrew Chan and Myuran Sukumaran, convicted of drug trafficking in Bali, Indonesia.
[12] McMahon, along with a team of lawyers including Todung Mulya Lubis have helped their clients prepare clemency requests, and has launched judicial reviews in an attempt to overturn their death sentences.
[14] In a comment to SBS Dateline, McMahon stated "they help Indonesians in prison become better people, the idea of killing two young men who do that day in and day out is just ridiculous.”[15] McMahon was named Victorian Australian of the Year in 2016 for his work as a barrister, human rights advocate and fierce opponent of the death penalty.