In October 2004, Shellenberger and his colleague Ted Nordhaus, both long-time environmental strategists, authored a controversial essay, "The Death of Environmentalism: Global Warming Politics in a Post-Environmental World." The essay argues that environmentalism is conceptually and institutionally incapable of dealing with climate change and should "die" so that a new politics can be born. The essay was widely discussed among liberals and greens in Salon.org, Grist.org , and The New York Times. In October, 2007, Houghton Mifflin published Nordhaus and Shellenberger's From the Death of Environmentalism to the Politics of Possibility(Houghton Mifflin, 2007). Break Through is an argument for a positive, "post-environmental" politics that abandons the environmentalist focus on nature protection for a new focus on creating a new economy.

After the failure of climate legislation in the U.S. Senate for the third time in June 2008, Time Magazine named Nordhaus and Shellenberger one of its 32 Heroes of the Environment calling Break Through "prescient" for its prediction that climate policy should focus not on making fossil fuels expensive through regulation but rather on making clean energy cheap, an argument the authors reiterated in an op-ed for the Los Angeles Times.

In 1996 Shellenberger co-founded Communication Works, a progressive public relations firm. By 2001, now "California's largest public interest communications firm", it merged with Fenton Communications, "the country's largest progressive PR and advertising agency". Communication Works worked on a wide range of campaigns, from challenging Nike over its labor practices in Asia, to saving the Headwaters Redwood forest.

In 2002 Shellenberger co-founded the public relations firm Lumina Strategies. Its clients included Global Exchange, Americans United for Affirmative Action, the Ford Foundation, and the Sierra Club. In 2004 Lumina was contracted by the Venezuela Information Office to lobby for the Venezuelan government and improve Hugo Chavez's image in the United States.

In 2005 Shellenberger and Nordhaus co-founded American Environics, whose clients include AARP, Earthjustice, the Ford Foundation, and the Nathan Cummings Foundation. He and Nordhaus also co-founded the Breakthrough Institute, a think tank that works on energy and climate change, health care, social inequality, and human rights.