Signer is the son of Marjorie B. Signer, a communications director, and Robert Signer, a newspaper assignment editor. He graduated from Washington-Lee High School in Arlington, Virginia, and magna cum laude from Princeton University.
He earned a Ph.D. in political science from the University of California, Berkeley, and a J.D. from the University of Virginia School of Law, where he was a Clerk at the Legal Aid Justice Center and Research Assistant to Professors A. E. Dick Howard and Michael Klarman. He was president of the Law Democrats, and co-founder of the UVA Chapter of the American Constitution Society. At UVA, he founded the UVA Coalition for Progress on Race, and went on to co-found the Center for the Study of Race and Law.
Signer is the author of Becoming Madison: The Extraordinary Origins of the Least Likely Founding Father (PublicAffairs 2015), a book about leadership and statesmanship that is also an intellectual and psychological biography of young James Madison and his rivalry with his nemesis Patrick Henry in the ratification of the U.S. Constitution. Signer wrote Demagogue: The Fight to Save Democracy from Its Worst Enemies (2009). He has published articles, essays, and book reviews in the University of Richmond Law Review, Corporate Counsel, The Washington Post, The New Republic, and the Daily Beast.
In 2006, he wrote an article on progressive American exceptionalism, titled "City on a Hill", in the inaugural issue of Democracy: A Journal of Ideas. Signer teaches nonfiction writing at Politics and Prose, a bookstore in Washington, D. C.
Signer is the founder and managing principal of Madison Law & Strategy Group, PLLC, where, since 2010, he has practiced corporate and regulatory law. He is co-chair of the Business Law Section of the Charlottesville-Albemarle Bar Association. He also chaired the Pro Bono Committee of the Young Lawyers Conference of the Virginia State Bar.
A voting rights attorney, he was statewide director for the 2004 program directed by the Democratic National Committee. In 2010, he traveled to Panjshir Province, Afghanistan, as a member of a USAID-sponsored mission to monitor Afghanistan's parliamentary elections. He founded and co-chaired the New Electoral Reform Alliance for Virginia.
Signer is chair of the Emergency Food Network, president of the Fifeville Neighborhood Association, and a member of the steering committee of the West Main Street Redevelopment Project in Charlottesville. He is a former member of the Board of Directors of the Center for National Policy. He is a principal and former board member of the Truman National Security Project, and is co-chair of the New Dominion Project PAC, a Virginia-based political action committee.
In the 2008 elections, Signer was foreign policy advisor to the John Edwards for President campaign. He was later senior strategist on the 2008 Congressional campaign of Tom Perriello. Signer was senior policy advisor at the Center for American Progress, and later that year worked with John Podesta on President-Elect Barack Obama's State Department Transition Team.
In 2009, Signer was a candidate for the Democratic nomination for Lieutenant Governor of Virginia, receiving only 21% of the vote.
From 2009 to 2013, Signer was an appointee by Governor Tim Kaine to Virginia's Board of Medicine. He was a member of the finance committee for Terry McAuliffe for governor, and later served as chair of Governor-elect McAuliffe's Transition Council on Homeland Security. Earlier in his career, he was legislative aide to then-Delegate Creigh Deeds. Signer created an Advisory Council on Innovation and Technology to link stakeholders in the Charlottesville technology sector. Governor Terry McAuliffe appointed him to the Council on Virginia's Future.
On January 4, 2016, Charlottesville City Council elected Signer as the city's new mayor, succeeding outgoing mayor Satyendra Huja. Signer took office the same evening.
As mayor, Signer worked with the city council to create a Blue Ribbon Commission on Race, Memorials, and Public Spaces to address controversies over Confederate statues in Charlottesville. Charlottesville also hired the city's first African-American police chief during Signer's tenure. During Signer's tenure as mayor, Charlottesville was named by Entrepreneur as the #4 City in the U.S. for entrepreneurship.
During Signer's tenure, the city council created an Open Data policy, and required agencies to register voters to vote online.
Signer, who is Jewish, lives in Charlottesville with his wife, Emily Blout, and their twin sons.