Halle began his career in the field of international negotiations, serving in the diplomatic secretariat of the Conference on Security and Cooperation in Europe. In that capacity, he was associated with the negotiation of the Barcelona Convention on the Mediterranean Environment, one of the first regional environmental conventions ever adopted. 

Thereafter, he spent five years with UNEP, working in its Policy Planning Unit and on the global State of the Environment report published 10 years after UNEP’s establishment. While at UNEP, he worked with WWF and IUCN on the World Conservation Strategy. The World Conservation Strategy fundamentally changed the approach to nature conservation, abandoning the earlier notion that conservation and development were in opposition to one another and embracing the notion that they are essential components of sustainable development. 

Halle then moved to WWF International, serving for three years as conservation assistant to HRH The Prince Phillip (a past president of WWF) and helping to establish and direct the WWF program in China. In 1984, he moved to IUCN to establish the Conservation for Development Centre – IUCN’s first involvement with developing countries. Halle directed the Centre for seven years, before a three-year stint setting up IUCN’s fundraising system and a final four years establishing its Global Policy and Partnerships program.