William K. Reilly is an American conservationist and former head of the U.S.  Environmental Protection Agency.  He is considered the "first professional environmentalist" to run this organization since its founding in 1970.

Reilly began his federal environmental career in 1970, when he was appointed to President Richard Nixon's Council on Environmental Quality.  In 1972, he was made Director of the Task Force on Land use and Urban Growth.  The next year, Reilly became President on the Conservation Foundation.  Under his leadership, this non-profit research group grew into an international environmental protection organization, which joined with the World Wildlife Fund in 1985, with Reilly continuing on as President.  The group soon boasted an annual budget of $35 million, as well as 600,000 members.

At this point, Reilly became an active critic of President Ronald Reagan's environmental policy, writing articles on such topics as pollution, wetlands, and rainforests.

As an EPA administrator under President George Bush, Reilly took a strong and aggressive leadership position on many environmental issues. These included law enforcement, toxic waste, urban smog, fuel efficiency, recycling, global warming, and ozone layer depletion.  Reilly also was instrumental in the revising of the Clean Air Act of 1970.  The result was a much stronger, forceful, and effective law.