Caplan has been a political activist all his life as a member of the CCF and its successor, the NDP. He was campaign manager in a series of election campaigns, both federal and provincial, including those for Ontario leader Donald C. MacDonald and national party leader David Lewis. A lifelong friend of David's son Stephen Lewis, he was also a close advisor and senior strategist to Lewis prior to and during his career as leader of the Ontario New Democratic Party in the 1970s. After a period serving as co-ordinator of Toronto's health advocacy unit from 1990 to 1982, Caplan returned to political work. From 1982 to 1984, Caplan was federal secretary of the New Democratic Party, and was national campaign manager for the 1984 general election.

In 1982, Caplan was a candidate for the federal NDP nomination in Broadview...Greenwood after Bob Rae vacated the seat to move to provincial politics. Caplan was upset in his bid for the nomination by Lynn McDonald, who defeated Caplan on the third ballot by a margin of 309 votes to 248. McDonald went on to keep the seat for the NDP in the ensuing by-election.

In 1985, Caplan was co-chair of the Task Force on Canadian Broadcasting Policy by then-Prime Minister Brian Mulroney. For many years he remained vocal in defence of public broadcasting, some of it with the lobby group Friends of Canadian Broadcasting.

He worked as a political columnist for the Toronto Star from the mid-1980s for a number of years, and was often on television and radio as a political pundit supporting the NDP. He was a member of the long-running Thursday morning Pundits' Panel on CTV Television's morning program, Canada AM. He continues to contribute opinion pieces to various Canadian newspapers including the Star and The Globe and Mail. Following the 1988 federal election, he co-authored Election : the issues, the strategies, the aftermath with Liberal strategiest Michael Kirby and Tory strategist Hugh Segal.

From 1993 to 1995, Caplan was co-chair of Ontario's Royal Commission on Learning having been appointed to the position by Ontario Premier Bob Rae. Prior to the 1999 Ontario election he served as the Director of Research for the Ontario NDP caucus.

In 1999, with his study of the Rwandan genocide, Caplan returned full-time to African matters. After completing the report on the genocide, he founded and became the co-coordinator of Remembering Rwanda, an international movement of volunteers dedicated to commemorating the tenth anniversary of the genocide. He has remained active around issues related to Rwanda and genocide prevention in general, frequently speaking and writing about both.

In 2001, he was named by the United Nation’s Special Coordinator for Africa as a member of the senior experts’ team undertaking an evaluation of the UN’s New Agenda for the Development of Africa in the 1990s. He has also acted as a consultant for the Economic Commission for Africa, UNICEF, WHO and the African Union.

He served, as well, as volunteer chair of the International Advisory Board for the University of Toronto Centre for International Health's HIV/AIDS Initiative, and is part of the small team that supports Stephen Lewis in his work as UN Special Envoy for HIV/AIDS in Africa.

Caplan has denounced the Roman Catholic Church on what he says is the notorious role of the institution in setting the stage for, enabling and ultimately participating in the genocide in Rwanda.

'President Obama needs a refresher course on Africa' (2009) Published in Pambazuka News[2]

'Abdelrazik's next challenge' (2009) Published in Pambazuka News[3]

'Another opening, another show' (2009) Published in The Globe and Mail[4]

Obama in Africa: A Major Disappointment (2009) Published in The Nation[5]

Caplan, G (2008) The Betrayal of Africa, Groundwood Books

Caplan, G (2005) Why We Must Never Forget the Rwandan Genocide in Manji, F and Burnett, P (eds) African Voices on Development and Social JusticeEditorials from Pambazuka News, Oxford: Pambazuka Press [6]

Caplan, G (2005) Some Reminders of the International Response to the Crisis in Manji, F and Burnett, P (eds) African Voices on Development and Social JusticeEditorials from Pambazuka News, Oxford: Pambazuka Press [7]

Caplan, G (1970) The elites of Barotseland,1878-1969: a political history of Zambia's Western Province, London: Hurst