His current research examines rationales for energy efficiency policies, the limits of cost-benefit analysis in climate policy, and the role of behavioral economics in energy and environmental policy. He has been co-author of two books on the deregulation of electricity markets and has analyzed constitutional requirements for compensation for public use of private land. He has studied privacy and environmental law enforcement related to remote-sensing satellites and is working on the roles of prizes in technological innovation and on assessing liability rules for space launches. Brennan also addresses issues in antitrust law, telecommunications policy, copyright, and the philosophy of economics. He has presented his research to numerous government, professional, and academic institutions in the U.S. and internationally, including Austria, Australia, England, France, Germany, Ireland, Mexico, New Zealand, Sweden, and Uzbekistan.

Brennan is also a professor of public policy and economics at the University of Maryland–Baltimore County and also has taught at George Washington Univer¬sity. He was an economist for the Antitrust Division of the U.S. Department of Justice from 1978 to 1986 and was senior economist for industrial organization and regulation on the staff of the White House Council of Economic Advisers in 1996–1997. From 2003 to 2005, he served as a staff consultant to the director of the Bureau of Economics of the Federal Trade Commission. He spent 2006 in Ottawa as the T.D. MacDonald Chair in Industrial Economics at the Canadian Competition Bureau. He is a co-editor of Economic Inquiry and on the editorial boards of the Journal of Regulatory Economics, Information Economics and Policy, Communications Law and Policy, and the International Journal of the Economics of Business.