Thomas is the Chairperson of the Coalition for Free and Fair Elections (BERSIH), a coalition of 56 non-governmental organisations (NGOs) that focuses on electoral and democratic reforms in Malaysia.
In BERSIH he has served as the Southern Vice-Chair and Treasurer from 2013 to 2016 before his election as the Chair in 2018. He was active in voters registration, organising peaceful protests and spearheaded the Delineation Action & Research Team (DART) of Bersih to prepare the public to object to any unfair delineation of electoral boundaries by the Election Commission.
As a social activist he has also initiated several notable human rights projects like Freedom Fund that helped activists and citizens who were being persecuted by the previous government for expressing views that were deemed critical of the government. He also founded Citizens Against Enforced Disappearances (CAGED) to monitor the disappearances of Pastor Raymond Koh and the others.