Guillaume Colin joined FIACAT in September 2006 as FIACAT's representative before the United Nations after having worked with Radio France Internationale (RFI) and Amnesty International's Africa programme in London. He then created the post of FIACAT representative before the African Commission on Human and Peoples' Rights (ACHPR) and run, from Paris and Bobo Dioulasso, a three-year programme to improve prison conditions in Burkina Faso. Based on this field experience, Guillaume set up FIACAT's programme to support the ACATs in abolishing the death penalty in sub-Saharan Africa in 2012.

In 2018, he became FIACAT's Executive Director.

At the same time, he is a lecturer in the Master “Solidarity and International Action” at the Faculty of Social and Economic Sciences (FASSE) of the Catholic Institute of Paris (ICP), where he teaches project management. He is also the Treasurer of the World Coalition Against the Death Penalty since November 2014.