Denis is President of the Bullitt Foundation, an environmental philanthropy organization located in Seattle. During the Carter Administration, Denis was Director of the federal Solar Energy Research Institute (now the National Renewable Energy Laboratory). From 1983 to 1988, he was an adjunct professor of engineering and human biology at Stanford University. A graduate of Stanford Law School, Hayes also practiced law for many years in Silicon Valley.
 
In 1970, Denis was National Coordinator of the first Earth Day in 1970. He continues to chair the board of Earth Day Network, which now has annual activities in 184 countries. Hayes also has been awarded the John Muir Award by the Sierra Club, the Charles Greeley Abbot Award by the American Solar Energy Society, and the Joseph Wood Krutch Award by the Humane Society. Time magazine has profiled Hayes as one of its "Heroes of the Planet;" and the National Audubon Society included him in its list of the 100 Environmental Heroes of the twentieth century.