Penniman was named Executive Director in 2014 when Fund For The Republic and Americans for Campaign Reform merged to form Issue One.
Penniman was executive director of the Huffington Post Investigative Fund, which he founded with Arianna Huffington in 2009. Supported by large foundations and The Huffington Post, the operation established a nonprofit newsroom of journalists who reported on a variety of topics, most notably the financial crisis of 2007-2008 and its aftermath. The fund was profiled in media publications like the American Journalism Review and the Columbia Journalism Review. In 2011, it merged with the Center for Public Integrity.
Penniman founded the American News Project and served as the Washington director of the Schumann Center for Media and Democracy, where he worked closely with broadcaster Bill Moyers. Moyers and Penniman first met in 1999, when Penniman was running a national grassroots organization called the Alliance for Democracy, which focused primarily on campaign finance reform.
From 2005-2006, Penniman was the publisher of The Washington Monthly magazine. Before that he was the executive editor of a progressive news and opinion website called TomPaine.com perhaps best known for the "op ads" it regularly ran on the opinion page of the New York Times.
He has served on multiple nonprofit boards and advisory boards, including the Homeless Empowerment Project, which publishes Spare Change News, the Center for Responsive Politics and the Roosevelt Institution.
In 2016, Penniman and co-author Wendell Potter published Nation on the Take: How Big Money Corrupts Our Democracy And What We Can Do About It. Nation On The Take details the history of money in politics and exposes the effects of the influence industry and political money on policy making and everyday Americans. The New York Journal of Books stated, "Nation on the Take is breathtaking in the level of detail for Washington K Street transgressions and the national landscape of politics and money. If you are still on the sidelines for your personal campaign to restore our democracy, this book will incite a riot in your conscience and illustrate some things we as a people can do to take back our country. This book is no less than a clarion call to the American people to rise up and release these ties that bind." The book was additionally praised, on its cover, by historian Doris Kearns Goodwin, who stated "There could be no more important or timely book." And New York Times columnist Nicholas Kristof referred to it in one of his pieces as an "eye-opening new book about money in politics."
- Joseph, Andrew (2012-01-03). "New Group Formed To Limit Political Money - Influence Alley". Influencealley.nationaljournal.com. Retrieved 2012-01-13.
- "Nation On The Take". Nation On The Take. Retrieved 2016-03-23.
- "a book review by Roberta E. Winter: Nation on the Take: How Big Money Corrupts Our Democracy and What We Can Do About It". www.nyjournalofbooks.com. Retrieved 2016-04-11.
- "About | Represent.Us". End corruption. Defend the Republic. Retrieved 2016-11-02.
- Board of Directors, Everglades Foundation
- Pulitzer Report Contains Lots of Clues, St Louis Journalism Review, May 1999, by Charles L Klotzer