He has served as policy advisor and writer for many international politicians and leaders, including Pierre Trudeau, François Mitterand, Edward Heath, Julius Nyerere, Olof Palme, Willy Brandt, Jimmy Carter and HRH Prince El Hassan of Jordan.

He  has also been an economic consultant for agencies in more than 30 countries. Quilligan is a co-founder of Global Commons Trust where he is working with colleagues to develop a Secretariat to provide research, support and publicity for a new international commission on the global commons. He has also launched a lobbying effort, Commons Action for the United Nations (CAUN), which has been successful in introducing the concepts of the commons into UN discussions and documents. Over the past several years, Quilligan has published a series of groundbreaking articles on the global commons in Kosmos Journal.