In 1988 he was named Minister of the Environment, and in 1990 founded the green Ecology Generation party.

In 1968, Lalonde was a member of Union Nationale des Étudiants de France (UNEF), the French national students' union, and chairman of UNEF-Sorbonne, which brought France to a standstill with protests and riots.

Later Lalonde, with David McTaggart, an activist involved in the Greenpeace protests against French nuclear tests at Mururoa atoll, helped create the confrontational strategies of boarding ships at sea in the 1970s.

In July 1973, he was arrested by the French Navy during protests against nuclear tests in Mururoa, along with General Jacques Pâris de Bollardière, priest Jean Toulat and writer Jean-Marie Muller.

In 2007 Brice Lalonde was appointed French Ambassador for Climate Change and was heavily involved in multilateral climate change negotiations until the end of 2010, when UN Secretary-General Ban Ki-moon appointed him Assistant Secretary-General, Executive Coordinator of the United Nations Conference on Sustainable Development (Rio+20), which was held in Rio de Janeiro in June 2012.