William Mebane was born in San Antonio, Texas where his father was a successful petroleum geologist. After graduation at the University of Texas, he worked for Ray Geophysical Company as a systems analyst and subsequently for Pexamin a small oil company. In 1965 he married Donata Francescato, an Italian community psychologist. William graduated from Harvard Business School in 1972, participating in seminars with Dennis Meadows at MIT. After meeting Aurelio Peccei and Umberto Colombo of the Club of Rome, he joined Montedison Research in Milan, where he headed the Technological Forecasting Unit, working on long-term energy and social modeling. With Giuseppe Colasanti, he published Intellectual Unemployment, a System Dynamics study of Italian Labor Market.

In the 1980s he consulted to ENEA (Italian National Agency for New Technologies, Energy and Sustainable Economic Development) in the field of energy savings, later heading the Department of Energy Efficiency. In 1982 he assisted in the drafting of the first Italian law promoting solar and energy savings. Regional consulting centers were opened, and emphasis was given to collaboration with Italian industry in the setting of efficiency standards and labeling for household appliances in the EU, which resulted in more than halving the average unit energy consumption. Now he consults to organizations facilitating sustainable development and lives in Rome, Italy.