He was a secretary general of International Alert and an advisor to the Reconciliation programme of the Triple R. Rupesinghe had experience in mediation and conflict resolution work.
One of Rupesinghe's core concerns was the brutality he witnessed during the ethnic riots of 1958 in Sri Lanka, where a close Tamil friend and his family were killed.
During 1973–1977, under the patronage of the Sri Lanka Freedom Party, Rupesinghe formed two major weekly newspapers, Janavegaya and Janavegam (both meaning 'people's power'), which embodied a social movement for change.
The organisation continued the work of the human rights advocate and first secretary general of Amnesty International, Martin Ennals, who received the Nobel Prize.
During his tenure, Rupesinghe was in the august company of IA Board members like Archbishop Desmond Tutu, Nobel Laureates and other distinguished citizens of the global human rights community.
Under his leadership, International Alert became one of the largest NGOs in the world with a budget of approximately UK£5 million a year, a staff of 80, and field programmes in over 15 countries.
International Alert has been involved in mediation and conflict resolution in Sri Lanka, the Philippines, Fiji, Sierra Leone, Burundi, Rwanda, the Caucasus region, and the Russian Federation.
It made a decisive intervention in the conflict in Sierra Leone and facilitated the mediation process with the foreign minister of Côte d'Ivoire, the UN, the OAU, and the Commonwealth secretariat.
As chairman of the FCE, Rupesinghe provided policy support for the Sri Lankan peace process by writing articles and issue papers and participating in seminars and TV opinion programmes.
He contributed to early warning systems for the EU and the OAU, and sought to influence policy debate in the UK, Canada, Norway, Sweden, the Netherlands, Finland, Denmark, Germany, and Japan, where his efforts led to the formation of the Centre for Preventive Diplomacy.
Rupesinghe also served as a member of the board on the United Nations Research Institute for Social Development (UNRISD)'s Project on War Torn Societies.
He recently released his collected works under the titles Waging Peace 2002-2008, Preventive Diplomacy, Early Warning, Conflict Resolution and Transformation Vols.