Swinnen was ambassador in Kigali from 1990 to 1994. He is known for sending Brussels early warnings of what became the upcoming Rwandan genocide while ambassador in Rwanda. In 1992, he cabled Brussels that the Interahamwe militia had taken part in the carefully planned killings of around 300 Tutsis in southeastern Rwanda's Bugesera District.[5][6][7][8][9][10] In January 1994, several months before the Rwandan genocide, he warned Brussels about calls on the radio for the extermination of the Tutsi.[11] He warned that as long as the United Nations Assistance Mission for Rwanda did not intervene, arms would be distributed to the Interahamwe.[12][13]
He was successively ambassador in The Hague (1997-2002), Kinshasa (2004–2008) and Madrid (2009-2011).
Swinnen has also been a diplomatic advisor to Prime Minister Jean-Luc Dehaene.[14]
Since 2015, he is chair of the board of the Brussels-based International Peace Information Service, an independent think tank that works on peace, sustainable development and human rights.[15]